Two possibilities are able to explain the phenotype that occurs in patients and in heterozygous KI/+ mice: (1) Kcnd3 F227del is a gain‐of‐function mutation that exerts a dominant‐negative effect to interfere with the normal functioning of the WT protein; and (2) the ataxia phenotype is associated with haploinsufficiency, in which the 50% reduction in the normal protein is insufficient for complete cellular functioning. The gene discussed is KCND3; the disease is cerebellar ataxia.