A floxed region containing exon 8 was excised via the expression of cre recombinase in vitro. The deleted region encoded the heme-binding domain. RT-PCR analysis of total renal RNA identified the presence of a truncated transcript in homozygous mutant mice. The absence of active vitamin D3 in serum samples was identified by an radioimmunoassay and indicates that the aberrant transcript present in homozgous mutant mice is non-functional.