In the process of creating a loxP-flanked allele, a tandem duplication of exon 1 and 2 with an frt-flanked neo cassete and a 5' loxP site inserted into intron 2 was knocked into the gene. The authors state that this allele is hypomorphic. Northern blot analysis confirmed reduced transcript levels in the lungs and the absence of transcript in the liver and placenta of E17.5 embryos. Enzymatic assays confirmed decreased activity in the adult lungs, liver and E15.5 placenta.