This allele is derived from Hoxb4tm2Nrm. Embryos from heterozygous Hoxb4tm2Nrm intercrosses were injected with a cre expression plasmid to remove the floxed neo cassette in the first coding exon. The deletion leaves behind a 78 bp insertion containing a loxP site in the first exon. The 78 bp insertion contains a stop codon in-frame with the Hoxb4 transcript.