CtBP1 and the related protein CtBP2 are characterized as C-terminal binding proteins of adenovirus E1A, and they preferentially associate with the E1A via a 5 amino acid motif, PLDLS, to repress E1A induced oncogenesis and cellular transformation.CtBP1 is expressed from embryo to adult, but CtBP2 is mainly expressed during embryogenesis. During skeletal and T cell development, CtBP1 and CtBP2 associate with the PLDLSL domain of deltaEF1, a cellular zinc finger-homeodomainprotein, and thereby enhances deltaEF1-induced transcriptional silencing. In addition, CtBP complexes with CtIP, a 125 kDa protein that recognizes distinctly different protein motifs from CtBP. CtIP binds to the BRCT repeats within the breast cancer gene BRCA1 and enables CtBP to influence BRCA1 activity.