Exon 4, which encodes the homeodomain of the protein product, was floxed by the insertion of a loxP site on one end and a loxP site and frt-flanked neo cassette on the other end. This allele was used to generate Isl1tm2Sev and Isl1tm2.1Sev. Immunohistochemistry confirmed reduced protein expression in the spinal cord.