AbstractMutations in CLCN1, the gene encoding the ClC‐1 chloride channel in skeletal muscle, lead to myotonia congenita. The effects on the intramembranous channel forming domains have been investigated more than that at the intracellular C‐terminus. We have performed a mutation screen involving the whole CLCN1 gene of patients with myotonia congenita by polymerase chain reaction (PCR), single‐strand conformation polymorphism studies, and sequencing. Two unrelated patients harbored the same homozygous G‐to‐T mutation on the donor splice site of intron 17. This led to the skipping of exon 17, as evidenced by the reverse transcriptase PCR. When the exon 17–deleted CLCN1 was expressed in Xenopus oocytes, no chloride current was measurable. This function could be restored by coexpression with the wild‐type channel. Our data suggest an important role of this C‐terminal region and that exon 17 skipping resulting from a homozygous point mutation in CLCN1 can lead to recessive myotonia congenita. © 2004 Wiley Periodicals, Inc. Muscle Nerve 29: 670–676, 2004