Proc. Natl. Acad. Sci. USAVol. 76, No. 10, pp. 5041-5045, October 1979Biochemistry

A new endogenous primate type C virus isolated from the Old Worldmonkey Colobus polykomos

(retrovirus/genetic transmission/molecular hybridization)

STEPHEN A. SHERWIN AND GEORGE J. TODAROLaboratory of Viral Carcinogenesis, National Cancer Institute, National Institutes of Health, Bethesda, NMaryland

Communicated by Robert J. Huebner, July 20, 1979

ABSTRACT A new, genetically transmitted retrovirus hasbeen isolated from the Old World monkey Colobus polykomos.This virus, designated CPC-1, is readily transmitted to both fe-line and human cells in culture. Nucleic acid hybridizationstudies reveal that there are 50-70 copies of the CPC-1 genomein colobus cellular DNA. Related virogene sequences can bedetected in the DNA of all other Old World monkeys, as wellas in the DNA of at least one ape species, the chimpanzee, in-dicating that this virus has been genetically transmitted in pri-mates for 30-40 million years. CPC-1 is partially related to thetype C virus previously isolated from stumptail monkeys(MAC-1). These two viruses have nucleic acid sequence ho-mology, antigenic crossreactivity in their major viral structuralprotein, and a very similar host range in vitro. CPC-1 and MAC-1therefore belong to the same class of genetically transmittedprimate type C viruses and, as such, represent the first examplein primates of analogous endogenous retroviruses isolated fromtwo distantly related species.

Genetically transmitted retroviruses have previously beenisolated from both New World and Old World monkeys (1-9).These isolates have been classified as type C or type D virusesbased on morphologic properties and can be subdivided intoseveral distinct classes according to both antigenic and nucleicacid hybridization criteria (8, 10). Endogenous type C viruseshave thus far been isolated from the New World species owlmonkey (Aotus trivirgatus) (7) and from Old World species,including stumptail monkey (Macaca arctoides) (8), rhesusmonkey (Macaca mulatta) (9), and baboon (Papio anubis)(1-3); endogenous type D viruses have been isolated from theNew World species squirrel monkey (Saimiri sciureus) (4, 5)and the Old World species langur (Presbytis obscurus) (6). Todate, there have been no convincing isolations of endogenoustype C or type D viruses from higher apes or humans.The endogenous type C viruses isolated from Old World

monkeys include two distinct groups: the numerous closelyrelated isolates from baboons (1-3) and the two highly similarisolates from macaques (8, 9). These two groups of virusesrepresent completely unrelated classes of genetically trans-mitted retroviruses and are both represented in multiple copiesin the genomes of all Old World monkeys (8-12). Whereas thebaboon virus group is readily isolated from various species andtissues of normal baboons (13), the two macaque viruses(MAC-1 and MMC-1) were both isolated in single, long-termexperiments after multiple attempts had failed to yield virus(8, 9).The Old World monkeys can be divided into two subfamilies,

the Cercopithecinae, which includes baboons, macaques, andseveral other species, and the Colobinae, which includes thecolobus and langur. In this paper, we report the isolation in a7-month cocultivation experiment of an endogenous type C

virus from the Old World monkey Colobus polykomos. Thisvirus is designated CPC-I for C. polykomos, type C, first isolate.CPC-1 is partially related by both antigenic and nucleic acidhybridization criteria to the previously isolated MAC-1 virusof stumptail monkeys. The virus is the first endogenous typeC virus isolated from the Colobinae and appears to be analogousto the type C viruses of macaques, though the two subfamilieshave been genetically separated for nearly 20 million years.

MATERIALS AND METHODSCell Culture and Virus Isolation. The cell lines used in these

experiments include a human carcinoma cell line A549 (14) thathas been unusually permissive for replication of various typeC viruses; rhesus lung DBS-FRhL-1 (15); African green monkeyVERO, bat lung Tb-1-Lu, and mink lung MV-1-Lu from theAmerican Type Culture Collection; canine thymus Cf2Th fromthe Naval Biomedical Research Laboratory (Oakland, CA);NIH/3T3 (16); and domestic cat embryo FEF from PeterFischinger (National Cancer Institute). A primary culture ofnormal kidnev fibroblast cells obtained from a specimen of C.polykomos was seeded with a variety of these indicator celllines. After 2 days, the cell mixtures were exposed to 5-bromodeoxyuridine (100 ,ug/ml) for 24 hr. The cells were thenmaintained in Dulbecco's modification of Eagle's mediumsupplemented with 10% fetal calf serum and were transferredevery 2 weeks by use of 0.1% trypsin in phosphate-bufferedsaline. Cultures were screened at 3- to 4-week intervals over an8-month period for the presence of type C virus by assayingsupernatants for reverse transcriptase activity.

Viral Polymerase Assays. Culture supernatants were assayedfor reverse transcriptase activity with a polyriboadenylatetemplate, an oligodeoxythymidilatel2-18 primer, and 0.6 mMmanganese chloride as described (17). Antiviral polymeraseantibody inhibition studies were performed by publishedmethods (18).

Viral Structural Protein Assays. Competitive radioimmu-noassays for the presence of different structural proteins in viralextracts were performed as described (18). An assay for the p26protein of MAC-1 virus (10) has been developed with a high-titered antiserum from a goat inoculated with whole, disruptedMAC-1 virus.

Molecular Hybridization. DNA and RNA were extractedfrom tissues and cell lines as described (11). 3H-Labeled DNAtranscripts of CPC-1 virus that had been disrupted with TritonX-100 were prepared in an endogenous reverse transcriptasereaction and partially purified by sedimentation in alkalinesucrose (19, 20). Transcripts ranging in size from 12 S to 16 Sin alkaline sucrose (2500-5000 nucleotides) were hybridizedto 2-4 mg of DNA or RNA per ml in the presence of 10 mMTris-HCl, pH 7.4/0.75 M NaCl/2 mM EDTA/0.05% sodiumdodecyl sulfate. Hybridizations were initiated by heating at100°C for 10 min; the mixtures were then incubated at 650C

5041

The publication costs of this article were defrayed in part by pagecharge payment. This article must therefore be hereby marked "ad-vertisement" in accordance with 18 U. S. C. §1734 solely to indicatethis fact.

Dow

nloa

ded

by g

uest

on

Dec

embe

r 20

, 202

0

5042 Biochemistry: Sherwin and Todaro

for various lengths of time. Hybrids were detected with thesingle-strand-specific S1 nuclease (21). Less stringent hybrid-ization conditions (12, 22) using a higher salt concentration (1.5M NaCl) and a lower hybridization temperature (60'C) wereperformed where indicated. Cot values were calculated ac-cording to the method of Britten and Kohne (23) and correctedto a monovalent cation concentration of 0.18 M (25). [Cot is theinitial concentration of nucleic acid (mol/liter) multiplied bytime (sec).]

RESULTSIsolation of CPC-1 Virus. A primary culture of normal

kidney cells was obtained from a specimen of C. polykomos.After 3 weeks in culture, these cells were seeded with ap-proximately 106 cells of a variety of indicator lines. Two dayslater the cell mixtures were exposed to 5-bromodeoxyuridinefor 24 hr at a concentration of 100 ,gg/ml. The cell mixtureswere subsequently tested at monthly intervals for the presenceof type C virus by assaying culture supernatants for reversetranscriptase activity. After approximately 7 months, the culturecontaining human carcinoma A549 cells became positive forreverse transcriptase activity. This viral activity was subse-quently designated CPC-1 for C. polykomos type C virus. Nosuch enzyme activity was detected in cultures containing batTb-i-Lu cells or canine Cf2Th cells (data not shown).The CPC-1 viral polymerase activity required manganese

as its divalent cation; no significant enzyme activity was de-tected when magnesium was substituted for manganese in thereaction mixture. CPC-1 polymerase activity was fully inhibitedby a broadly reacting antiserum to the polymerase of catRD-114 virus, a serum previously shown to inhibit the poly-merase of most type C viral isolates (data not shown). In theserespects, therefore, CPC-1 reverse transcriptase activity appearsto resemble that of a mammalian type C virus.CPC-1 Is an Endogenous Virus of the Colobus Monkey.

In order to determine whether CPC-1 is an endogenous retro-virus of colobus monkeys, a DNA transcript of the virus grownin the human carcinoma cells was prepared in an endogenousreverse transcriptase reaction and hybridized to DNA extractedfrom the tissues of colobus monkeys and other primates. Theresults of these experiments are shown in Table 1. Hybridizationwas detected by S1 nuclease digestion at two temperatures andsalt concentrations: "high stringency" conditions (65'C and 0.75M NaCl), at which only closely matched nucleotide sequenceshybridize, and "low stringency" conditions (60°C and 1.5 MNaCl), at which more distantly related hybrids can be detected(22). As shown in Table 1, at either high- or low-stringencyconditions, the CPC-1 DNA transcript hybridized almostcompletely (>90%) to the DNA extracted from colobus monkeytissues. There was no appreciable difference in the level ofhybridization to the DNA of the two different colobus speciesthat were tested. In contrast, there were much lower levels ofhybridization at either high- or low-stringency conditions tothe DNA of other primate species. These findings demonstratethat CPC-1 was derived from endogenous virogene sequencespresent in colobus monkey cell DNA.

As indicated in Table 1, the CPC-1 transcript hybridizedsignificantly to the DNA of other Old World monkey species.At high-stringency conditions there was approximately 10%hybridization to langur DNA and between 15 and 21% hy-bridization to the DNA of Old World monkeys belonging to theCercopithecinae subfamily (baboon, patas, mangabey, vervet,and macaque). Moreover, at low-stringency conditions, therewas a significant increase in the level of hybridization (29-34%)to the DNA of Cercopithecinae species. Thus, the CPC-1transcript easily detects partially related virogene sequences

Table 1. Hybridization of CPC-1 [3HJDNA transcripts to variousprimate cellular DNAs

% hybridization*650C, 0.75 M 600C, 1.5 M

Species NaCl NaCl

Old World monkeysColobinae

Colobus:C. polykomosC. guereza

LangurCercopithecinae

Stumptail monkeyPigtail macaqueBaboonPatasMangabeyVervet

ApesGibbonOrangutanChimpanzeeHumans

New World monkeysSquirrel monkeyHowler monkeyOwl monkey

ProsimianGalago

Nonprimates

9610010

181921201715

<1<1101

<1<12

<1

929716

333434303329

55

227

76

NT

8

Cat 3 8Sheep <1 6Squirrel <1 NTGuinea pig <1 5

* Hybridization was determined by S1 nuclease digestion with[3H]DNA transcripts of CPC-1 virus that were 12 S-16 S in size.Reactions were carried out to a Cot of >103 at the indicated tem-peratures and salt concentrations. NT, not tested.

in other Old World monkey species that are close relatives ofthe colobus, a further characteristic of a genetically transmittedvirus.The data in Table 1 also reveal that the CPC-1 transcript

hybridized at low levels to the DNA of chimpanzees, but notdetectably to the DNA of other higher primates, includinggibbons, orangutans, and humans. The hybridization tochimpanzee DNA was significantly higher than the nonprimatebackground levels at both high-stringency (10%) and low-stringency (22%) conditions. This suggests that chimpanzeesalso contain CPC-1-related sequences in their genome despitethe evolutionary divergence of colobus monkeys and chim-panzees, which occurred nearly 30-40 million years ago. Incontrast, under the same hybridization conditions, no relatedvirogene sequences could be detected in the DNA of the otherapes tested and of various New World monkeys, prosimians,and nonprimates.To further demonstrate that CPC-1 is an endogenous virus

of colobus monkeys, we performed experiments to determinethe number of copies of the CPC-1 genome in colobus DNA.Endogenous retroviruses are typically found in multiple copiesin the cellular DNA of the species of origin (25). Fig. 1 showsthe kinetics of hybridization of the CPC-1 transcript to colobuscellular DNA and to the cellular DNA of another Old Worldmonkey, the stumptail macaque. As shown, the Cotl/2 for thehybridization to colobus DNA is approximately 30. Because theC0t1/2 for the self-annealing of single-copy mammalian DNAunder identical reaction conditions is 1500-2000 (see Fig. 1),

Proc. Natl. Acad. Sci. USA 76 (1979)

Dow

nloa

ded

by g

uest

on

Dec

embe

r 20

, 202

0

Proc. Natl. Acad. Sci. USA 76 (1979) 5043

C

0

'M 60-

.C 40

20-

10" 10' lo2 103 104Cot, (moles X sec)/liter

FIG. 1. Hybridization of the [3HJDNA transcript of CPC-1 virusto colobus and stumptail monkey DNAs. The percent hybridizationwas determined by S1 nuclease digestion and is normalized. The ac-

tual percent hybridization for the homologous control was 94%. TheDNAs tested were extracted from colobus (0), stumptail monkey (0),humans (A), and sheep (A). The self-annealing of 3H-labeled non-

repetitive stumptail monkey cellular DNA is shown for reference(x).

one can estimate that there are approximately 50-70 copies ofthe CPC-1 genome in colobus cellular DNA. Moreover, theCotl/2 for the hybridization of the CPC-1 transcript to stumptailmacaque DNA is also approximately 30, although the finalextent of hybridization is much lower (24%). This suggests thatthere are 50-70 copies of a partially related viral genome in theDNA of the stumptail macaque. This is the expected result,assuming that CPC-I is an endogenous colobus virus and thatpartially related virogene sequences are present in the samecopy number in the DNA of other Old World monkeys.CPC-1 Is Related to MAC-I, the Endogenous Type C Virus

of Stumptail Macaques. CPC-1 was tested for relatedness toother previously isolated endogenous primate retroviruses byhybridizing the CPC-1 transcript to the RNA of these viruses.As shown in Table 2, the CPC-1 transcript hybridized 18% tothe RNA of MAC-1, the endogenous type C virus of stumptailmacaques, but did not hybridize to the RNA of other primatetype C viruses isolated from baboons and owl monkeys or oftype D viruses isolated from langurs and squirrel monkeys.

Table 2. Hybridization of CPC-1 [3H]DNA transcripts to RNA ofendogenous primate retroviruses

% hybridization with[3H]DNA transcript

Species of of:torigin Viral RNA* CPC-1 MAC-1

Colobus CPC-1, type C 100 29Stumptail monkey MAC-1, type C 18 100Baboon M28, type C 1 2Langur PO-1-Lu, type D <1 2Owl monkey OMC-1, type C <1 1Squirrel monkey M534, type D 2 3

* Viral RNA was obtained from heterologous cell lines producingthese viruses: CPC-1, MAC-1, and M28 were grown on human A549cells; PO-1-Lu and OMC-1 on bat Tbl-Lu cells; and M534 on

DBS-FRhL-1 cells.t The percent hybridization, as determined by S1 nuclease digestion,

is normalized. The actual percent hybridization for the homologouscontrol was >90%.

100 0.01 1.0 100Competing protein, Mg

FIG. 2. Competition radioimmunoassays for viral structuralproteins. (A) Interspecies assay for Old World monkey type C viralp30 with goat antibody to MAC-1 and 1251-labeled M28 p30. (B)Species-specific assay for MAC-1 p26 with goat antibody to MAC-1and 125I-labeled MAC-1 p26; (C) species-specific assay for M28 p30with goat antibody to M28 and 125I-labeled M28 p30. The viral ex-tracts used as competing antigens were colobus CPC-1 (0), stumptailmacaque MAC-1 (0), baboon M28 (-), and owl monkey OMC-1(A).

These results demonstrate nucleic acid sequence homologybetween CPC-1 and MAC-1. This is, in fact, confirmed by thereciprocal hybridization of the MAC-1 transcript to CPC-1 viralRNA, which resulted in 29% hybridization (Table 2).On the basis of the apparent nucleic acid homology between

CPC-1 and MAC-1, it seemed probable that at least some of thestructural proteins of these two viruses would share a degreeof antigenic relatedness. This hypothesis was tested in a seriesof competition radioimmunoassays for known viral structuralproteins. Fig. 2 shows the results of some of these experiments.The CPC-1 viral extract was highly positive in the interspeciesassay, which detects both M28 baboon viral p30 and MAC-Imacaque viral p26 (Fig. 2A), thus confirming that CPC-1 canbe detected in an assay for Old World monkey type C virus.CPC-1 extract was also positive in the species-specific assay forMAC-1 p26, an assay which does not score M28 p30 (Fig. 2B).In this assay, the CPC-1 viral protein crossreacted in the com-petition assay but had a different slope from that of the ho-mologous MAC-1 control. This indicates that CPC-I containsa viral structural protein antigenically related to, but never-theless distinct from, the p26 protein of MAC-1. In contrast,neither CPC-1 nor MAC-1 viral extracts were detectable in thespecies-specific assay for baboon M28 p30 (Fig. 2C).The relationship between CPC-1 and MAC-1 was further

explored by comparing the in vitro host range of these two vi-

Table 3. Comparison of host range of CPC-1 and MAC-1virus isolates

Supernatant reversetranscriptase activity

Indicator cell 6 wk after infection with:*lines CPC-1 MAC-1

Human A549 1028.1 230.4Bat Tbl-Lu 3.2 1.9Rhesus DBS-FRhL-1 5.9 3.9Canine Cf2Th 5.2 4.4Cat FEF 51.1 212.3Mink Mv-1-Lu 4.2 4.4Green monkey VERO 2.7 2.0Mouse NIH/3T3 2.4 5.3

* Indicator cell lines were infected with 1.0 ml of filtered supernatantof A549 cells producing either CPC-1 or MAC-1 viruses. Culturesupernatants were subsequently assayed for reverse transcriptaseactivity. Data are expressed as cpm X 10-3 of [3H]dTMP incorpo-rated. Values significantly above background are italicized.

Biochemistry: Sherwin and Todaro

Dow

nloa

ded

by g

uest

on

Dec

embe

r 20

, 202

0

5044 Biochemistry: Sherwin and Todaro

ruses. Table 3 shows the results of an experiment in which avariety of indicator cell lines were infected with filtered su-pernatants of A549 cultures producing CPC-1 and MAC-1. Bothof these viruses can be easily transmitted within 6 weeks afterinfection to human A549 cells and to cat FEF cells. In contrast,the growth of both viruses appears to be restricted in a varietyof other mammalian cell lines. These results suggest a possiblesimilarity in the viral envelope glycoproteins of CPC-1 andMAC-I to the extent that this viral protein is an important de-terminant of host range in vitro. Thus, CPC-1 and MAC-1 mayhave a similar envelope glycoprotein as well as an antigenicallyrelated p26 structural protein. Both of these experimentalfindings are in keeping with the observed nucleic acid homol-ogy between the two viral genomes.

DISCUSSIONIn this paper, we report the isolation and initial characterizationof a new endogenous retrovirus of primates from the Old Worldmonkey colobus. This virus has been designated CPC-1 forColobus polykomos type C virus. CPC-I virus contains reversetranscriptase activity antigenically related to previously isolatedmammalian type C viruses and can be reproducibly transmittedto both human A549 and cat FEF cells. Nucleic acid hybrid-ization studies have shown that the CPC-1 genome is presentin 50-70 copies in colobus monkey cellular DNA and thatpartially related sequences are present in a similar copy numberin the DNA of other Old World monkeys. CPC-1 is thereforean endogenous type C virus of the colobus monkey, and ho-mologous virogene sequences appear to have been geneticallypreserved in various other Old World monkey genomes.

In terms of both nucleic acid sequence homology and anti-genic crossreactivity, CPC-1 is partially related to MAC-I, theendogenous type C virus isolated from stumptail macaques. Avirus virtually identical to MAC-I has also been isolated froma Macaca mulatta cell line and designated MMC-1 (9). BothCPC-1 and MAC-I show no appreciable sequence homologyto any of the other previously isolated endogenous primateretroviruses, including M28 baboon type C virus, OMC-1 owlmonkey type C virus, PO-i-Lu langur type D virus, and M534squirrel monkey type D virus. In addition, CPC-1 and MAC-1appear to have an identical host range in vitro. CPC-1 andMAC-1, then, are related to each other but are distinct from allof the other classes of endogenous primate retroviruses. CPC-1and MAC-I, therefore, represent separate isolates from the sameclass of endogenous primate type C viruses and, as such, com-prise the first example of two analogous retroviruses isolatedfrom primate species separated in evolution by nearly 20 mil-lion years.

Fig. 3 summarizes the evolutionary relationships of thoseprimate species from which endogenous retroviruses have beenisolated. As shown, of the six genetically transmitted primateviruses isolated to date, the only two viruses related to each otherare the type C viruses obtained from the colobus and macaque.Virus isolates homologous to the endogenous viruses of owlmonkeys, squirrel monkeys, langurs, and baboons have not beenobtained from distantly related primates.CPC-1 and MAC-1 viruses are also similar in their pattern

of hybridization to the DNA of other primate species. BothCPC-1 and MAC-1 viral transcripts hybridize more to the OldWorld monkeys belonging to the Cercopithecinae subfamilythan to the langur and more to chimpanzees than to other apesand humans (Table I and ref. 8). The African Old Worldmonkeys (baboon, mangabey, patas, vervet, and colobus) andapes (chimpanzee and gorilla), show a considerably greaterdegree of homology to the African-derived baboon type C vi-ruses than do the Asian Old World monkeys (macaque and

PROSIMIANS

60 Myr

NEW WORLD MONKEYS/40

OWL MONKEY TYPE CSQUIRREL MONKEY TYPE D

OLD WORLD MONKEYS APES

/20/Myr\

COLOBINAE ERCOPITHECINAE

COLOBUS TYPE C MACAQUE TYPE C? ~ BABOON TYPE C

LANGUR TYPE D ?

FIG. 3. Species of origins of the known endogenous retrovirusesof primates. Points of evolutionary divergence between primatefamilies and subfamilies are indicated in millions of years (Myr) andare taken from estimates based principally on the fossil record.

langur) and apes (gibbon, orangutan, and possibly humans) (12,26). In like fashion, the precise pattern of hybridization of theCPC-1 (and MAC-1) transcript to primate DNA may also bepartially explained in terms of the Asian or African origin ofdifferent primate species. Because CPC-1 is derived from anAfrican Old World monkey (colobus), the level of hybridizationshould be somewhat greater to African Old World monkeys,such as baboon, patas, and macaques (which range in Africaas well as Asia), than to a strictly Asian Old World monkey suchas the langur. Moreover, the level of hybridization to Africanapes such as chimpanzee should be greater than to Asian apessuch as the orangutan and gibbon. These are, in fact, the resultsobtained (Table 1). Thus, hybridization studies with CPC-1 viraltranscripts lend further support to the hypothesis that the extentof conservation of specific virogene sequences in the genomesof Old World monkeys and apes parallels the Asian or Africanhabitat of these primates or their ancestors.

It is important to emphasize that the isolation of CPC-1 viruswas the result of a single, long-term cocultivation experimentand that the culture had remained negative for viral polymeraseactivity until nearly 8 months had elapsed. Thus, the isolationof CPC-1 virus is another example of "low-frequency" virusactivation in primates (8). Other examples of low-frequencyvirus activation among previously isolated primate retrovirusesinclude MAC-1 stumptail macaque type C virus (8), MMC-1rhesus macaque type C virus (9), OMC-1 owl monkey type Cvirus (7), and PO-i-Lu langur type D virus (6). These viruseswere all isolated in single experiments lasting from 7 to 8months. The isolation of these viruses and of CPC-1 contrastsstrongly with the "high-frequency" activation of the endoge-nous baboon type C virus (1-3, 13) and endogenous squirrelmonkey type D virus (4,5). The latter viruses have been isolatedrepeatedly and rapidly from various tissues and on a wide rangeof host cells. Thus, it would now appear that a majority of pri-mate species are capable only of low-frequency virus activation.Further isolations of additional endogenous primate retrovi-ruses-including perhaps those of apes and humans-maytherefore require prolonged cocultivation. with unusuallypermissive host cells.We thank Linda Papke, Janis Koci, and Nancy Cope for their expert

technical assistance.

Proc. Natl. Acad. Sci. USA 76 (1979)

Dow

nloa

ded

by g

uest

on

Dec

embe

r 20

, 202

0

Proc. Natl. Acad. Sci. USA 76 (1979) 5045

1. Todaro, G. J., Tevethia, S. S. & Melnick, J. L. (1973) Interirology1,399-404.

2. Benveniste, R. E., Lieber, M. M., Livingston, D. M., Sherr- C. J.,Todaro, G. J. & Kalter, S. S. (1974) Nature (London) 248, 17-20.

3. Goldberg, R. J., Scolnick, E. M., Parks, W. P., Yakovleva, L. A.& Lapin, B. A. (1974) Int. J. Cancer 14,722-730.

4. Heberling, R. L., Barker, S. T., Kalter, S. S., Smith, G. C. &Helmke, R. J. (1977) Science 195,289-292.

5. Colcher, D., Heberling, R. L., Kalter, S. S. & Schlom, J. (1977)J. Virol. 23,294-301.

6. Todaro, G. J., Benveniste, R. E., Sherr, C. J., Schlom, J., Schid-lovsky, G. & Stephenson, J. R. (1978) Virology 84, 189-194.

7. Todaro, G. J., Sherr, C. J., Sen, A., King, N., Daniel, M. D. &Fleckenstein, B. (1978) Proc. Nati. Acad. Sci. USA 75, 1004-1008.

8. Todaro, G. J., Benveniste, R. E., Sherwin, S. A. & Sherr, C. J.(1978) Cell 13,775-782.

9. Rabin, H., Benton, C. V., Tainsky, M. A., Rice, N. R. & Gilden,R. V. (1979) Science 204, 841-842.

10. Bryant, M. L., Sherr, C. J., Sen, A. & Todaro, G. J. (1978) J. Virol.28,300-313.

11. Benveniste, R. E., Heinemann, R., Wilson, G. L., Callahan, R.& Todaro, G. J. (1974) J. Virol. 14, 56-67.

12. Benveniste, R. E. & Todaro, G. J. (1976) Nature (London) 261,101-108.

13. Todaro, G. J., Sherr, C. J. & Benveniste, R. E. (1976) Virology 72,278-282.

14. Lieber, M. M., Smith, B., Szakal, A., Nelson-Rees, W. & Todaro,G. J. (1976) Int. J. Cancer 17,62-70

15. Wallace, R. E., Vasington, P. J., Petricciani, J. E., Hopps, H. E.,Lorenzo, D. W. & Kadanka, Z. (1973) In Vitro 8, 333-341.

16. Jainchill, J. L., Aaronson, S. A. & Todaro, G. J. (1969) J. Virol.4,549-553.

17. Lieber, M. M., Sherr, C. J. & Todaro, G. J. (1974) Int. J. Cancer13,587-598.

18. Sherr, C. J., Fedele, L. A., Benveniste, R. E. & Todaro, G. J. (1975)J. Virol. 15, 1440-1448.

19. Rothenberg, E. & Baltimore, D. (1977) J. Virol. 21, 168-178.20. Benveniste, R. E. & Todaro, G. J. (1977) Proc. Nati. Acad. Sci.

USA 74, 4557-4561.21. Benveniste, R. E. & Scolnick,E. M. (1973) Virology 51,370-382.22. Kohne, D. E. (1970) Q. Rev. Biophys. 3, 327-375.23. Britten, R. J. & Kohne, D. E. (1968) Science 161, 529-540.24. Britten, R. J. & Smith, J. S. (1970) Carnegie Inst. Washington

Yearb. 68, 378-386.25. Benveniste, R. E. & Todaro, G. J. (1974) Nature (London) 252,

170-173.26. Benveniste, R. E. & Todaro, G. J. (1978) Arthritis Rheum. 21,

Suppl., S2-S16.

Biochemistry: Sherwin and Todaro

Dow

nloa

ded

by g

uest

on

Dec

embe

r 20

, 202

0