With a human myelin-basic-protein (MBP) cDNA used as a probe, the human MBP gene has been mapped to chromosome region 18q22-q23 by a combination of Southern hybridization to a panel of somatic-cell hybrid DNAs and in situ hybridization to metaphase chromosomes. Restriction-fragment-length polymorphisms (RFLPs) have also been identified with this probe in human DNA, by means of the restriction enzymes BamHI, PvuII, and PstI. In studies of informative families, the alleles of the BamHI and PvuII polymorphisms have been shown to segregate as Mendelian traits.