Dominant transmission of multiple uterine and cutaneous smooth-muscle tumors is seen in the disorder multiple leiomyomatosis (ML). We undertook a genomewide screen of 11 families segregating ML and found evidence for linkage to chromosome 1q42.3-q43 (maximum multipoint LOD score 5.40). Haplotype construction and analysis of recombinations permitted the minimal interval containing the locus, which we have designated "MCUL1," to be refined to an approximately 14-cM region flanked by markers D1S517 and D1S2842. Allelic-loss studies of tumors indicated that MCUL1 may act as a tumor suppressor. Identification of MCUL1 should have wide interest, since this gene may harbor low-penetrance variants predisposing to the common form of uterine fibroids and/or may undergo somatic mutation in sporadic leiomyomata.