Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease or Lou Gehrig’s disease, is a fatal disorder of unknown etiology. The disease involves degeneration of motor neurons, leading to paralysis, respiratory failure, and death within five years. A viral etiology for ALS has been suggested but never proven. Retroviruses have been considered because they cause motor neuron disease in mice, and HIV-1 and HTLV-1 cause ALS-like symptoms in humans. Sera from some ALS patients have been shown to contain elevated levels of reverse transcriptase, an enzyme found in retrovirus particles. RNAs encoding this enzyme have now been found in the brains of ALS patients, and their origin appears to be the human endogenous retrovirus HERV-K.