Insertion of a floxed neomycin resistance cassette into the second exon introduced a premature translation stop codon into the open reading frame. Pronuclear injection of pMC1-Cre into fertilized eggs carrying the Hoxb8tm1Mrc allele removed the neo cassette, leaving a single loxP site in the second exon. RT-PCR studies of brain detected a longer transcript produced by this allele than by the wild-type allele.