Here we report a Japanese family with amyotrophic lateral sclerosis (ALS) characterized by very rapid progression, high penetrance and an autosomal dominant mode of inheritance. The phenotype includes atrophy of sternocleidomastoideus muscles, bulbar involvement, weakness of neck muscles and proximal muscle atrophy. These clinical symptoms are reminiscent of myopathy. All patients examined had similar clinical symptoms, age at onset and disease duration. The proband was found to have mutation R521C in the FUS/TLS gene, and was diagnosed as having ALS6. Autopsy material was available from the mother of the proband and FUS-immunoreactive neuronal and glial cytoplasmic inclusions were observed in the anterior horn of the spinal cord. While atrophy and weakness of the sternocleidomastoideus muscle is not emphasized in previous reports, this symptom may be a clinical hallmark of ALS6. (C) 2010 Elsevier B.V. All rights reserved.