Retroviral components of both exogenous and endogenous origins have been associated with nervous system diseases in both animals and humans. In the present study, the levels of transcripts from elements in the human endogenous retrovirus (HERV) W family were determined in muscle biopsies from patients with motor neuron disease (MND) and control subjects. Transcripts from the HERV-W element on chromosome 7q21.2 encoding syncytin and from the SOD1 gene were detected at elevated levels in biopsies from the most affected muscles from MND patients compared to biopsies from control individuals. According to a recent study, syncytin is expressed in microglia in normal brain and can be up-regulated in macrophages/microglia during inflammation. Although syncytin may have cytotoxic effects, it is therefore more likely that the present findings reflect a macrophage response in the muscles undergoing neurogenic atrophy than a primary pathogenetic event in MND.