A neomycin resistance cassette into exon 5 replaced part of the 5' untranslated region of the gene for the gamma2 isoform and the A2 region sequences up to the codon for amino acid 19 (nt 81-342). This disruption is predicted to selectively prevent the synthesis of the gamma2 isoform. RNase protection analysis of E10.5 embryos indicated that the mRNA encoding the gamma2 isoform was mutated in this allele, and that the wild-type transcript for the gamma1 isoform was transcribed.