DDB1 was originally identified as a heterodimeric complex with DDB2 and plays an accessory role in nucleotide excision repair. DDB1 also constitutes an E3 ubiquitin ligase complex together with Cul4A and Roc1 and acts as an adaptor, suggesting its multiple roles beyond DNA repair. We have generated a conditional DDB1-knockout mutant using a chicken B lymphocyte line DT40. Doxycycline-induced DDB1 depletion caused a severe growth defect followed by apoptotic cell death. Flow cytometric analyses revealed that cell cycle progression is initially retarded at all phases and subsequently impaired at S phase along with the appearance of sub-G1 population. Similarly, DDB1-knockdown in human U2OS cells by small interfering RNA exhibited a loss of clonogenic activity and perturbed cell cycle progression. These results demonstrate that the DDB1 gene is indispensable for cell viability in higher vertebrates and this conditional DDB1-knockout clone would be highly useful for the functional analysis of DDB1. (C) 2007 Elsevier Inc. All rights reserved.