A Search for Clues to the Cause of Amyotrophic Lateral Sclerosis

To the Editor.—  The cause of amyotrophic lateral sclerosis (ALS) is unknown. The unusual geographic distribution of this disease with focuses in Guam and the Kii Peninsula of Japan suggests an environmental trigger,¹ but no consistent hypothesis has yet been established.Recently, we described² a motor neuron disease in wild mice (Mus musculus) with clinical and pathologic similarities to ALS; the endogenous type C RNA tumor virus of these wild mice was established as the etiologic agent of this neurologic disease.^2,3 We conducted an epidemiologic study to seek evidence of a similar viral etiologic agent for ALS.An ALS registry was established in the Los Angeles area with the help of neurologists and the ALS Foundation who were contacted and asked to report prevalent and all future cases of ALS. From this source, 139 possible cases of ALS were identified. Of these, eight were subsequently excluded as