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The KCNQ5 gene, widely expressed in the brain, encodes a voltage-gated potassium channel (Kv7.5) important for neuronal function. Here, we report a novel KCNQ5 intragenic duplication at 6q13 spanning about 239 Kb of genomic DNA, identified by array comparative genomic hybridization (array-CGH). The duplication was found in heterozygosity in an adult patient affected by mild intellectual disability with history of absence epilepsy in adolescence, with no EEG nor MRI alterations. By in vitro analyses we demonstrated that this copy number variation (CNV) led to an aberrant transcript with exon 2–11 skipping and a premature stop codon causing, most likely, haploinsufficiency. The Kv7.5 channel plays an important role in the regulation of M-type current and afterhyperpolarization conductances which contribute to neuronal excitability. A recently published paper described KCNQ5 missense mutations in individuals with intellectual disability and treatment-resistant epilepsy that were thought to act through either loss-of-function or gain-of-function mechanisms, associated in both cases with altered neuronal excitability. In the case reported here, we showed that no functional protein can be produced from the allele involved by the intragenic duplication. This evidence strongly supports the hypothesis of KCNQ5 haploinsufficiency, which could lead to altered neuronal excitability, thus contributing to seizure susceptibility and intellectual disability. [ABSTRACT FROM AUTHOR]