CtIP is a nuclear protein conserved among vertebrates that was discovered as a cofactor of the transcriptional corepressor CtBP. CtIP also interacts with the tumor suppressors such as BRCA1 and the pRb family members through binding sites that are frequently mutated in human cancers. CtIP is a target for BRCA1-dependent phosphorylation by the ATM kinase induced by DNA double strand breakage. CtIP plays a role in DNA-damage-induced cell cycle checkpoint control at the G2/M transition. Homozygous inactivation of the Ctip gene causes very early embryonic lethality during mouse development. The Ctip(-/-) embryo cells are arrested in G1 and do not enter S phase. Depletion of Ctip in established mouse embryo fibroblasts arrests cells in G1 and results in an accumulation of hypophosphorylated Rb and the Cdk inhibitor p21, suggesting that CtIP is also a critical regulator of G1/S transition of the cell cycle. The Ctip gene contains a mononucleotide (A9) repeat and one of the alleles is mutated at a high frequency in colon cancers with microsatellite instability. The Ctip(+/-) mice develop multiple types of tumors suggesting that haploid insufficiency of Ctip leads to tumorigenesis. Among the various tumor types observed in Ctip(+/-) heterozygous mice, large lymphomas are prevalent. Recent studies raise the possibility that Ctip may itself be a tumor susceptibility gene and suggest that it might be important for the activities of tumor suppressors BRCA1, pRb family proteins and Ikaros family members.