Nuclear localization and cell cycle-specific expression of CtIP, a protein that associates with the BRCA1 tumor suppressor

The BRCA1 tumor suppressor has been implicated in a diverse spectrum of cellular processes, including tran-scriptional regulation, DNA repair, and cell cycle check-point control. CtIP was recently identified as a protein that associates with BRCA1 and two other nuclear fac-tors, CtBP1 and Rb1. To understand the functions of CtIP, we have evaluated its biological properties with respect to those of BRCA1. Our results show that CtIP, like its associated factors, is predominantly a nuclear protein. A subset of the endogenous pool of CtIP polypeptides exists in a protein complex that includes both BRCA1 and the BRCA1-associated RING domain protein (BARD1). At the protein level, CtIP expression varies with cell cycle progression in a pattern identical to that of BRCA1. Thus, the steady-state levels of CtI