To identify the chromosomal localizations of the multiple sclerosis (MS) genes, we conducted a genomewide linkage analysis using eighteen affected families. A MS gene is linked to markers located in the 19q13.3 region (multipoint lod-score = 2.1). Apolipoprotein E (ApoE) gene, located in this region, is an excellent candidate gene for MS because the ApoEe4 allele is acting as a severity allele in the disease.