CBS and Myotonia: The complex mutation observed in this patient resulting in a premature stop codon affected the structure of α-helices P (encompassing amino acids 541–551), Q (555–571), and R (576–585), as well as the 2 intracellular C-terminal cystathionin-β-synthase (CBS) segments (609–876), in addition to the P-Q and Q-R and the C-terminal loops, all of which play key roles in the proper functioning of CLCN1. This importance is underscored by ~110 mutations located in this region that have been reported in humans, where amino acid alterations have led to congenital myotonia (25).