The causes of sporadic motor neuron diseases, primary lateral sclerosis (PLS) and amyotrophic lateral sclerosis (ALS) are unknown. Genes have been identified for some forms of familial motor neuron diseases. We don't know whether genes also play a role in sporadic motor neuron disease, for example through risk-factor genes or by the interaction of multiple genes as a complex genetic disorder. Identification of genetic contributions to sporadic motor neuron diseases requires analysis of DNA from patients.