We describe a novel genetic variant mouse that exhibited ataxia and male sterility, named the AMS mouse. It arose in autoimmune-prone MRL/lpr strain and putative ams mutation showed an autosomal recessive inheritance pattern. Clinical symptoms were first discernible at approximately 21 days of age and consisting of subtle sway of the trunk followed by failure to maintain still posture and appearance of abnormal walk, but no further worsening was noted with advancement of age. The abnormal motor coordination was ascribed to almost complete loss of Purkinje cells of the cerebellum. The cell loss in the Purkinje cell layer began before onset of ataxia and rapidly progressed towards near-complete loss by 6 weeks of age. Another symptom was male sterility due to severe oligozoospermia associated with cellular degeneration during spermatic differentiation in the seminiferous tubules. Thus, the effects of the genetic variation were apparent in two different organs after the development of their basic histological structures, and degeneration and loss of particular cell types in these two tissues produced overt clinical symptoms. Genetic pleiotropism, provided that the nature of genetic variation is of a single gene mutation, is discussed.