Alexander disease, a rare fatal disorder of the central nervous system, causes progressive loss of motor and mental function. Until recently it was of unknown etiology, almost all cases were sporadic, and there was no effective treatment. It was most common in an infantile form, somewhat less so in a juvenile form, and was rarely seen in an adult-onset form. A number of investigators have now shown that almost all cases of Alexander disease have a dominant mutation in one allele of the gene for glial fibrillary acidic protein (GFAP) that causes replacement of one amino acid for another. Only in very rare cases of the adult-onset form is the mutation present in either parent. Thus, in almost all cases, the mutation arises as a spontaneous event, possibly in the germ cell of one parent.