Any neurodevelopmental disorder in which the cause of the disease is a mutation in the KCNH1 gene. Variants in KCNH1 cause significant neurodevelopmental disabilities that lie along a phenotypic spectrum ranging from non-syndromic to syndromic. The most common phenotypes associated with variants in KCNH1 include intellectual disability, seizures, hypotonia, absence or hypoplasia of nails, and gingival enlargement. Hypoplastic terminal phalanges of fingers and toes, proximal placement and long thumb, and long toes present less frequently. [ http://www.ncbi.nlm.nih.gov/pubmed/25915598 http://www.ncbi.nlm.nih.gov/pubmed/25420144 https://clinicalgenome.org/affiliation/40006/ http://www.ncbi.nlm.nih.gov/pubmed/33811134 ]