In an attempt t o verify the nature of amplification events at band q I 3 on chromosome I I we surveyed the amplification status of ten molecular markers specific for this region (GSTP, SEA. D I I S97, D I I S 146, BCL I, PRAD IICCND I , HSTIFGF4. INTZIFGf3, E M S / . and DI IS833E) 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 I I q I 3 can cover large portions of DNA and are consistent with the existence of several cores of amplification. One imporrant core seems t o be, as previously described, centered around PRADIlCCNDI; 57 tumors (14.7%) showed amplification at PRADIICCNDI either alone (one rumor) or along with amplification of BCLl or INT2/FGF3. The level of amplification of PRAD IICCND I sometimes exceeded that of surrounding markers. Three additional amplification events occurring independently of amplification of PRADIICCNDI were also detected. Centromeric to BCLI. probes to DI IS97, and D I I S I46 detected amplification in 60 tumors ( I 5.4%) and were often the only amplified markers. Telomeric to INTZIFGF3, DI 15833E was found amplified alone in ten tumors, and it was the most amplified marker in another six cases. A t a shorter distance of INTZIFGF3, E M S / was the only amplified marker in two tumors, with a level of amplification that could exceed that of PRAD IICCND I and D I I S833E. Our data thus suggest the existence of four independent amplified regions within band I I q I 3