Part of intron 1 and the first part of the coding sequence in exon 2, which includes the initiation sequence and the codons for the first 44 amino acids, were replaced with an IRES and a betageo fusion gene. This removes the highly conserved QUA1 domain in the encoded protein. Absence of transcription of the endogenous gene was demonstrated by RT-PCR. Beta-galactosidase is expressed from this allele.