The 2.0 kb BamH1 restriction fragment corresponding to a cDNA insert encoding the human c-sis PDGF A chain and nucleotide sequences homologous to the v-sis gene was isolated from plasmid pSM-1. An identical polymorphism was noted using the 1.2 kb PstI fragment or the 1.0 kb PstI/XbaI fragment isolated from the v-sis sequence subcloned in the plasmid pV-sis. HindIII identifies a single bi-allelic polymorphism with bands at 22.6 kb and 19.4 kb. Co-dominant segregation was demonstrated in 1 family. This polymorphism is not easily detected unless the restricted DNA is separated on 0.6-0.8% agarose gels. Resolution was optimal if gels were run until the bromophenol blue tracking dye had migrated 14 cm from the origin and Southern blotting was performed under alkaline conditions.