The entire coding region was replaced with a construct containing lacZ and neo. The lacZ was inserted immediately after the start codon, and thus placed under control of the endogenous promoter. Northern blot analysis revealed a lack of transcript in fibroblasts isolated from homozygous mutant embryos stimulated by IFN-gamma. An absence of expression was further confirmed by RT-PCR analysis of total embryonic RNA. The lacZ expression pattern was observed to be the same in whole mount homozygous embryos as it was in heterzygotes.