In an attempt to probe the significance of HST and INT-2 gene amplification in human breast carcinomas, we have surveyed the amplification status of five molecular markers located on the long arm of chromosome 11 (BCL-1, HST, INT-2 & SEA on 11q13, and ETS-1 on 11q23) in a population of 297 mammary tumors. ETS-1 was rarely amplified and always independently from the other proto-oncogenes. Concerning band q13: (i) 50 tumors (approximately 17%) were co-amplified for BCL-1, HST & INT-2; (ii) in 3 cases, amplification extended to the SEA gene; (iii) in 6 carcinomas, BCL-1 was the only amplified marker. The fact that we never observed amplification of HST & INT-2 independently of BCL-1, which in turn can be amplified solely, suggests the presence, between HST/INT-2 and BCL-1, of a genetic element which could be important in the development of a subset of mammary tumors.