Mutations in the gene coding for the skeletal muscle Cl(-) channel (CLCN1) lead 
to dominant or recessive myotonia. Here, we identified and characterized CLCN1 
mutations in Costa Rican patients, who had been clinically diagnosed with 
myotonic dystrophy type 1 but who were negative for DM1 mutations. CLCN1 
mutations c.501C>G, p.F167L and c.1235A>C, p.Q412P appeared to have recessive 
inheritance but patients had atypical clinical phenotypes; c.313C>T, p.R105C was 
found in combination with c.501C>G, p.F167L in an apparently recessive family 
and the c.461A>G, p.Q154R variant was associated with a less clear clinical 
picture. In Xenopus oocytes, none of the mutations exhibited alterations of fast 
or slow gating parameters or single channel conductance, and mutations p.R105C, 
p.Q154R, and p.F167L were indistinguishable from wild-type (WT). p.Q412P 
displayed a dramatically reduced current density, surface expression and exerted 
no dominant negative effect in the context of the homodimeric channel. 
Fluorescently tagged constructs revealed that p.Q412P is expressed 
inefficiently. Our study confirms p.F167L and p.R105C as myotonia mutations in 
the Costa Rican population, whereas p.Q154R may be a benign variant. p.Q412P 
most likely induces a severe folding defect, explaining the lack of dominance in 
patients and expression systems, but has WT properties once expressed in the 
plasma membrane.