A construct containing a codon-improved cre (icre) recombinase-neomycin cassette was generated from a BAC containing the Crisp4 locus. The linearized vector targeted the third exon of Crisp4 in ES cells and homologys recombination resulted in a deletion causing a frameshift that disrupted endogenous gene expression. This allele expresses icre from the Crisp4 promoter, specifically in the epididymis.