A construct containing a nonsense mutation at codon 245 (in exon 6 and orthologous to human position 250) and a floxed neo cassette in intron 6 was incorporated at the endogenous locus via homologous recombination. Transient transfection of ES cells with cre excised the neo cassette, leaving a single loxP site. Northern blot analysis of total hepatic RNA showed a reduced abundance of normal size transcript in homozygous mutant mice, putatively due to nonsense-mediated decay. Encoded protein was undetected by Western blot analysis.