In an attempt to verify the nature of amplification events at band q13 on chromosome 11 we surveyed the amplification status of ten molecular markers specific for this region (GSTP, SEA, D11S97, D11S146, BCLI, PRAD1/CCND1, HST/FGF4, INT2/FGF3, EMS1, and D11S833E) in a panel of 389 primary breast carcinoma DNA samples. Eighty-eight tumors (23%) showed at least one of these markers amplified, but in a majority of the cases amplification encompassed more than one of the tested loci. Our data confirm that amplicons at 11q13 can cover large portions of DNA and are consistent with the existence of several cores of amplification. One important core seems to be, as previously described, centered around PRAD1/CCND1; 57 tumors (14.7%) showed amplification at PRAD1/CCND1 either alone (one tumor) or along with amplification of BCL1 or INT2/FGF3. The level of amplification of PRAD1/CCND1 sometimes exceeded that of surrounding markers. Three additional amplification events occurring independently of amplification of PRAD1/CCND1 were also detected. Centromeric to BCL1, probes to D11S97, and D11S146 detected amplification in 60 tumors (15.4%) and were often the only amplified markers. Telomeric to INT2/FGF3, D11S833E was found amplified alone in ten tumors, and it was the most amplified marker in another six cases. At a shorter distance of INT2/FGF3, EMS1 was the only amplified marker in two tumors, with a level of amplification that could exceed that of PRAD1/CCND1 and D11S833E. Our data thus suggest the existence of four independent amplified regions within band 11q13 in breast cancer. Genes Chrom Cancer 9:42-48 (1994). © 1994 Wiley-Liss, Inc.