Amyotrophic lateral sclerosis (ALS) is a rapidly progressive, adult-onset motor neuron disease that arises as a dominantly inherited trait in ∼10% of ALS cases. Mutations in one gene, cytosolic Cu/Zn superoxide dismutase (SOD1), account for ∼25% of familial ALS (FALS) cases. We have performed a genetic linkage screen in 16 pedigrees with FALS with no evidence for mutations in the SOD1 gene and have identified novel ALS loci on chromosomes 16 and 20. The analysis of these genes will delineate pathways implicated as determinants of motor-neuron viability and provide insights into possible therapies for ALS.