A sequence fragmenet encompassing 234 bp of the promoter and exons 1 and 2 was replaced with a PGK-neo cassette inserted by homologous recombination. Endogenous transcript was undetected by Northern blot analysis of homozygous mutant mice following partial hepatectomy. Western blot analysis of whole liver extracts from quiescent and 1 to 8 hour posthepatectomy homozygous mutant mice showed an absence of normal protein.