{{B}}pAssAgE thrEE{{/B}}

thrEE yEArs Ago, rEsEArChErs AnnounCED thE DisCovEry oF humAn gEnEs thAt wErE CApABlE oF turning orDinAry CElls into mAlignAnt onEs. thE nEws mEt with somE skEptiCism.ExpErts AskED how A singlE gEnE CoulD CAusE suCh A DrAmAtiC ChAngE. why DoEs CAnCEr tAkE yEArs or EvEn DECADEs to DEvElop iF it is CAusED By suCh A simplE AnD DirECt proCEss in lAst wEEk’s issuE oF thE, thrEE rEsEArCh tEAms AnswErED thosE quEstions By sEtting Forth A nEw moDEl For unDErstAnDing thE rolE oF onCogEnEs in CAnCEr. EACh group FounD thAt it DoEs in FACt tAkE morE thAn A singlE gEnE to proDuCE CAnCEr in normAl CElls. tEAms At m. i. t AnDColD spring hArBor lABorAtory on long islAnD, n. y. , rEportED thAt thEy CoulD inDuCE CAnCEr in normAl rAt CElls only By insErting At lEAst two typEs oF onCogEnE into thE CElls. "A、singlE onCogEnE proDuCED somE ChAngEs, But not CAnCEr," ExplAinED molECulAr Biologist roBErt wEinBErg oF mit. it took two gEnEs ACting CoopErAtivEly to proDuCE A tumor. in othEr CAsEs, it might tAkE thrEE or morE. A、British tEAm ConFirmED thE multistEp CAnCEr sCEnArio By showing thAt A pArtiCulAr onCogEnE CAusED A tumor in hAmstEr CElls only iF thEy hAD First BEEn ExposED to A CArCinogEniC ChEmiCAl. thE ChEmiCAl AlonE AnD thE onCogEnE AlonE DiD not CAusE CAnCEr, Both wErE nECEssAry. whilE thE DisCovEry hAs no immEDiAtE impliCAtion For trEAtmEnt oF CAnCEr, it hElps ExplAin why thE DisEAsE DEvElops slowly AnD why its inCiDEnCE risEs with AgE. "EvEn iF onE pArt oF thE proCEss oCCurs," sAys wEinBErg, "you might not hAvE thE sEConD stEp For AnothEr 20 yEArs. "

CElls injECtED with A singlE CAnCEr gEnE ______.A．wErE unAFFECtED ComplEtEly unDEr suCh CirCumstAnCEs
B．only oCCAsionAlly DEvElopED CAnCEr
C．DEvElopED tumor But not CAnCEr
D．wErE ChAngED But DiD not BEComE mAlignAnt