An Hprt minigene replaced 0.5kb of sequence containing 121bp of the proximal promoter region and exon 1. Northern blot analysis of E13.5 embryos demonstrated that no mRNA containing exon 2 and 3 sequences is detectable in homozygous mutant mice. Western blot analysis on brain tissue from homozygous mutant mice showed that the protein is absent. (J:45775)