Mice carrying Tg(Hba1-Gfp)1Ew were treated with ENU. A single base pair, T to C mutation, was detected in intron 1 at the a canonical splice site for exon 1/intron 1. Both the wild-type and a higher molecular weight mutant PCR product are detected in heterozygous mice. The aberrant splice product results in the incorporation of an additional 1,160 nucleotides. The resulting protein is predicted to incorporate 79 additional amino acids followed by a premature stop codon and be subject to nonsense mediated decay. Western blot analysis of testis lysates from heterozygous mice revealed reduced protein expression.