We have previously shown that human piebaldism results from mutations of the KIT gene, which encodes the receptor for the mast/stem cell growth factor and is located in chromosome segment 4q12. Using DNA of a patient with piebaldism, mental retardation, and multiple congenital anomalies associated with a 46,XY,del(4) (q12q21.1) karyotype, we carried out quantitative Southern blot hybridization analyses of the KIT gene and the adjacent PDGFRA (platelet-derived growth factor receptor alpha subunit) genes. The patient was hemizygous for both the KIT and PDGFRA genes, indicating that both of these genes are included within the deleted region. Therefore, deletion of the KIT and PDGFRA genes may account for the piebald phenotype in this patient.