To explore the function of TRIP13 in mammals, we generated mice with a gene trap-disrupted allele, Trip13RRB047 (Figure 1C; abbreviated as Trip13Gt). Heterozygotes were normal in all respects, but homozygotes were present at ∼2/3 the expected ratio from intercrosses between heterozygotes (91 Trip13 +/+, 183 Trip13Gt/+, and 61 Trip13Gt/Gt). Since >90% of prewean mice that died were mutant homozygotes, this discrepancy is apparently due to a partially penetrant lethality. Most surviving Trip13Gt/Gt animals were grossly normal. However, homozygotes that were semi-congenic (N4) on the C57BL/6J strain were often markedly smaller and/or had kinked or shorter tails (Figure 2A and 2B).