Germ cell nuclear factor (GCNF), an orphan nuclear receptor, is essential for mouse embryogenesis. GCNF specifically binds to a DR0 response element via its DNA binding domain (DBD) in vitro and functions as a repressor of gene transcription. To further study the role of GCNF during embryogenesis, we have employed a Cre/loxP strategy and generated a line ofGCNF mutant mice (GCNFlox/lox) in which the 243-base pair DBD-encoding exon has been deleted in the germline. However, the ligand binding domain (LBD) of GCNF is still expressed at the mRNA and protein levels in theGCNFlox/lox mice.GCNFlox/lox mice die at 9.5–10.5 days postcoitum. The tailbuds of these mutant embryos protrude outside the yolk sac. Expression of Oct-4 in the somatic cells ofGCNFlox/lox embryos at 8.25 days postcoitum was not silenced as in the GCNF+/+ embryos. Therefore, GCNFlox/lox mice phenocopy theGCNF−/− mice. Our results indicate that the DBD is essential for the function of GCNF during early mouse embryogenesis, and that the LBD does not mediate any function independent of the DBD at this stage of embryonic development. Our results also suggest that GCNF is indeed a transcriptional factor that represses gene transcription mediated via its DBD.