CLCN1 and Thomsen and Becker disease: A high impact 8-base pair (bp) deletion across the end of exon 3 and intron 3 of the chloride voltage-gated channel 1 (CLCN1) gene was identified using whole genome sequencing.<h4>Treatment and outcome</h4>Phenytoin treatment was initiated at 3 mg/kg po q24 h and resulted in long-term improvement.<h4>Clinical relevance</h4>This novel mutation within the CLCN1 gene is a cause of myotonia congenita in cats and we report for the first time its successful treatment.