A PGK-neo cassette replaced the portion of exon 4 encoding amino acids 153 through 160, part of an essential catalytic domain. Expression analysis of homozygous mutant mice showed an absence of normal transcript and additionally that alternate trancripts were not produced by cryptic splicing. Western analysis revealed a lack of protein in homozygous mutants, while casein zymography did not detect calpain activity in platelets.