Ye et al. (2001) reported the presence of a single point mutation in the CDK3 gene from several Mus musculus strains commonly used in the laboratory. This mutation results in the replacement of a conserved tryptophan (trp187) within kinase consensus domain IX with a stop codon. The protein predicted to be encoded by this allele was truncated near the T loop, which is involved in activation by CDK-activating kinase (CCNH). This mutation also deletes motif XI, known to be required for kinase function, and is therefore expected to generate a null allele. The mutation was missing in 2 wild-mice species. The data indicated that CDK3 is not required for M. musculus development and suggested that any functional role played by CDK3 in the G1/S-phase transition is likely to be redundant with another CDK.