KCND1 and neurodevelopmental disorder: Since the two remaining group 1 variants, p.Tyr61Cysfs∗31 and p.Arg99∗, which were not functionally studied, are predicted to result in complete LOF, either due to NMD of the respective mRNA or the inability of the encoded protein to form functional channels, the above data collectively suggest that all five identified group 1 KCND1 variants are pathogenic, impairing Kv4.1 channel function and resulting in an X-linked neurodevelopmental disorder with variable expressivity.