An Hprt mini gene replaced a portion of exon 4 and all of exons 5 and 6. While translation of the targeted allele may be possible, in which case a truncated protein lacking all but 161 of the 637 amino acids would be produced, Western blot analysis showed protein levels to be decreased by approximately 50% in heterozygous mice. An assay of endogenous enzyme activity correspondingly showed an approximate 50% reduction in hepatic tissue obtained from heterozygotes.