Structure of the human c-mos protooncogene in DNAs from breast tumors, leukemic cells, and lymphocytes from normal individuals was analyzed by restriction enzyme digestion and Southern blot. In 6 of 75 breast tumor DNAs, we found an EcoRI 5-kilobase extra band hybridizing with a human c-mos probe containing all of the sequences homologous to v-mos oncogene. This band was also found in lymphocyte DNA from 3 of these patients, indicating a restriction fragment length polymorphism. This polymorphism was not found in a series of 69 lymphocyte DNAs from the unaffected population. Moreover, 1 of 73 leukemic cell DNAs exhibited the 5-kilobase band. These results indicate that this rare polymorphism is significantly more frequently found in patients with breast cancer than in the rest of the population (P less than 0.05, by a chi 2 test with Yates correction.