The gene for the human platelet-derived growth factor (PDGF) A type receptor was assigned to the proximal long arm of chromosome 4 by using in situ hybridization. Of 141 labeled metaphase cells, 74 had grains over chromosome 4, with a distinct peak at bands q11-q12. The presence of the gene on chromosome 4 was also confirmed by hybridization to chromosome specific libraries. This places the PDGFA receptor gene in the same region of chromosome 4 as the KIT oncogene, another member of the PDGF growth factor receptor subfamily. The two other members of this gene family, the PDGFB receptor and the colony stimulating factor-1 (CSF1) receptor, are closely linked on the distal half of the long arm of chromosome 5.