Currently, various genes corresponding to numerous complex phenotypes, such as spinocerebellar ataxias, spastic paraplegias and amyotrophic lateral sclerosis, are associated with SPG7, SPG11, PNPLA6, KIF1C and SETX, and they may be inherited as both autosomal dominant and recessive traits (KIF1A, REEP2, AFG3L2, SETX). In clinical practice, it becomes problematic whether the identified gene variant should be classified as corresponding to a new phenotype or if it “fits” the patient’s genotype consistent with the previous clinical diagnosis. This evidence concerns the gene KIF1A and cerebellar ataxia.