Replacement of exons 22-23 with a PGK-neo cassette via homologous recombination specifically disrupted the aspartyl beta-hydroxylase activity of one of 3 different protein products from this gene. Western blot of various tissues from homozygous mutant animals did not detect protein product and real-time PCR assay did not detect transcripts containing exons 22 and 23. In vitro and in vivo enzyme assays demonstrated protein extracts from homozygous mutant animals lacked aspartyl beta-hydroxylase activity.