Disruption of out-of-frame exon 5 with a neomycin cassette via homologous recombination resulted in the splicing of exon 4 to exon 6. This resulted in the introduction of a premature translational stop codon. Homozygous mutant mice had no detectable beta-mannosidase activity in any tissue, and heterozygotes had apporoximately half the enzyme activity of wild-type. Alpha-mannosidase activity was increased in the mutants compared to wild-type, especially at older ages.