Exons 3-15 of the gene were replaced by the open reading frame of beta-galactosidase followed by a loxP flanked neo cassette. The loxP flanked neo cassette was removed in vivo by cre-mediated deletion. RT-PCR analysis confirmed the absence of mRNA in thymocytes and MEFs. Absence of protein product was confirmed by western blot of thymocyte extracts derived from homozygous mice.