A rare EcoRI restriction fragment length polymorphism (RFLP) in the 3' end of the human c-mos locus has been identified in DNA from patients with breast tumors, esophageal carcinomas and leukemias. Until now, this RFLP has not been found in normal populations, suggesting that its presence may reflect some cancer susceptibility. To characterize this RFLP, we have isolated both alleles of the c-mos locus from DNA of a breast cancer patient and determined the nucleotide sequence of the polymorphic region. Our results show that this RFLP is due to a single nucleotide substitution (T instead of C), resulting in the disappearance of EcoRI site.