The Pax transactivation domain-interacting protein (PTIP) is a large nuclear protein with multiple BRCT domains that was identified on the basis of its interaction with transcription factors of the Pax and Smad families. To address the function of PTIP during mouse development, we generated a constitutive null allele. Homozygous PTIP mutants are developmentally retarded, disorganized, and embryonic lethal by day 9.5 of embryonic development (E9.5). PTIP mutant cells appear to replicate DNA but show reduced levels of mitosis and widespread cell death by E8.5. DNA damage appears to precede nuclear condensation at E7.5, suggesting a defect in DNA repair. Neither embryonic fibroblast nor embryonic stem cells from PTIP mutants proliferate in culture, suggesting a fundamental defect in cell proliferation. Trophoblast cells from PTIP mutants are more sensitive to DNA-damaging agents. Condensation of chromatin and expression of phospho-histone H3 are also affected in PTIP mutants, and this may underlie the inability of PTIP mutants to progress through mitosis. Given the role of BRCT domain proteins in DNA repair and cell cycle control, we propose that PTIP is an essential element of the cell proliferation machinery, perhaps by functioning in the DNA repair pathways.