A self-excising neomycin resistance cassette was inserted into exon 3 of the endogenous gene. The targeting construct was comprised of sequence encoding GFP followed by a floxed region containing a mouse testes-specific Ace promoter fused to the cre recombinase and neomycin resistance genes. The floxed region undergoes self-excision during spermiogenesis, resulting in the production of protein encoded by exons 1 and 2 and a 5' portion of exon 3 fused to GFP.