The homozygous deletion of exon 3 in the splicing regulator protein muscleblind1 (Mbnl1ΔE3/ΔE3) results in defective splicing of the chloride channel protein 1 (Clc-1) and reproduces clinical manifestations of myotonic dystrophy type 1 (DM1)12. The gene discussed is CLCN1; the disease is myotonic dystrophy type 1.