CTBP1 and infection: We also determined the effects of endogenous CtBP1 and CtBP2 on the E2F1 promoter activity by using A549 cells that were acutely depleted of both CtBP1 and CtBP2 by infection with lentiviral vectors that express shRNAs against CtBP1 and CtBP2 followed by short term drug selection.