We describe a maternally transmitted genomic-imprinting syndrome of mental retardation, hypotonia, and unique dysmorphism with elongated face. We mapped the disease-associated locus to ∼7.27 Mb on chromosome 8q24 and demonstrated that the disease is caused by a missense mutation in the maternal copy of KCNK9 within this locus. KCNK9 is maternally transmitted (imprinted with paternal silencing) and encodes K2P9.1, a member of the two pore-domain potassium channel (K2P) subfamily. The mutation fully abolishes the channel's currents-both when functioning as a homodimer or as a heterodimer with K2P3.1.
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