Chromosome locus 11q13 is frequently amplified in a number of human cancers including carcinoma of the breast where up to 15% carry this chromosomal abnormality. Originally 11q13 amplification was thought to involve a single amplicon spanning many megabases, but more recent data have identified four core regions within 11q13 that can be amplified independently or together in different combinations. Although the region harbors several genes with known or suspected oncogenic potential, the complex structure of the amplicons and the fact that 11q13 is gene-rich have made definitive identification of specific genes that contribute to the genesis and progression of breast cancer a difficult and continuing process. To date CCND1, encoding the cell cycle regulatory gene cyclin D1, and EMS1, encoding the filamentous actin binding protein and c-Src substrate cortactin, are the favored candidates responsible for the emergence of two of the four amplification cores.