Congenital Idiopathic Nystagmus (CIN) is genetically heterogeneous. Autosomal dominant, autosomal recessive and X-linked patterns of inheritance have been reported. Linkage analysis has suggested the existence of at least three distinct loci for both autosomal dominant and x-linked forms, although as yet no disease genes have been identified. The pathophysiological mechanisms underlying nystagmus are poorly understood and it is likely that insights may arise from finding and characterizing disease genes. If linkage experiments are used to find "nystagmus genes," their power will depend heavily on accurate phenotyping to avoid misdiagnosis due to masquerading conditions and phenotypic variations within pedigrees.