BACKGROUND Frequent allelic losses on the long arm of chromosome 13 in sporadic breast carcinomas suggest that a tumor suppressor gene(s) on 13q is involved in this type of carcinoma. The presence of a familial breast carcinoma susceptibility gene, BRCA2, and the retinoblastoma susceptibility gene (RB) on the same chromosomal arm implies that one or the other, or both, of these genes may be critically affected by those allelic losses. METHODS To investigate the possible involvement of BRCA2 and RB in sporadic breast carcinomas, the authors examined allelic losses in 246 breast carcinomas with 14 polymorphic microsatellite markers on 13q12.q14. RESULTS Allelic loss was observed in 95 of the 246 sporadic breast carcinomas (39%). Detailed deletion mapping identified two commonly deleted regions. The more proximal of these two segments was located in a 6-cM interval flanked by marker loci D13S289 and D13S267 and containing the BRCA2 gene; the more distal region was located in a 9-cM interval flanked by marker loci D13S328 and D13S172 and containing the RB gene. Allelic loss on 13q was found more frequently in tumors of the solid tubular histologic type (36 of 66; 55%) than in other types (52 of 146; 36%) (P = 0.0096). Furthermore, a significant association was observed between allelic loss on 13q and the absence of progesterone receptor (P = 0.0001). CONCLUSIONS The results indicate that BRCA2 and RB are independent targets of allelic loss and that inactivation of either of these genes may play a role in the development of some sporadic breast carcinomas, particularly those of the solid tubular type.