Events on different chromosomes alter the genes for the two peptides of platelet-derived growth factor in an osteogenic sarcoma. Platelet derived growth factor consists of dimers of partially homologous peptides (A and B) encoded by separate genes on different chromosomes. A clinically aggressive osteogenic sarcoma had both an amplified A-chain gene on chromosome #7 and a reduplication of all or part of chromosome #22 containing the B-chain (c-sis) gene. Other oncogenes on chromosomes #1, #2, #5, #6, #7, #8, #11, #12, #14, and #15 were not reduplicated or amplified. These observations are consistent with the hypothesis of autocrine stimulation of growth of some sarcomas in humans, and indicate that amplification and reduplication of related genes on different chromosomes can occur in tumors in vivo.