Fanconi anaemia (FA) is a DNA repair disorder characterized by cellular hypersensitivity to DNA cross-linking agents and extensive phenotypic heterogeneity. To determine the extent of genetic heterogeneity present in FA, a panel of somatic cell hybrids was constructed using polyethylene glycol-mediated cell fusion. Three new complementation groups were identified, designated FA(B), FA(C) and FA(D), and the gene defective in FA(C) which we have recently cloned was localized to chromosome 9q22.3 through in situ hybridization. These results suggest that mutations in at least four different genes lead to FA, a degree of genetic heterogeneity comparable to that of other DNA repair disorders.