Intrafamilial heterogeneity in hereditary motor neuron disease

Although there are varied inheritance patterns in motor neuron disease (MND), the phenotype of MND is reported to be constant within these families, ie, cases of amyotrophic lateral sclerosis or primary lateral sclerosis do not occur in pedigrees with cases of spinal muscular atrophy. We describe four pedigrees whose members diverged in the phenotype of MND expressed. The intrafamilial variation of phenotype suggests a similar pathogenesis for some of the varied types of familial MND and the need for careful inquiry of family history in all patients with MND.