Insertion of an ETn transposon disrupted sequences encoding D9, the ninth of twelve putative transmembrane domains. Northern analysis of skeletal muscle using a 5' rat Clcn1 cDNA probe detected aberrant size transcripts in homozygous mutant mice, but in the same tissues using a rat Clcn1 cDNA probe for sequences 3' to the D9 region, northern analysis detected no transcript.