Abstract: cDNA sequences derived from four novel genes associated with breast cancer are provided. In over about 60% of the cancer cell lines tested, RNA hybridizing with the cDNAs were substantially more abundant than in normal cells. Most of the cell lines also showed a duplication of the corresponding gene, which probably contributed to the increased level of RNA in the cell. However, for each of the four genes, there were some cell lines which had RNA overabundance without gene duplication. This suggests that the gene product is sufficiently important to the cancer process that cells will use several alternative mechanisms to achieve increased expression. The polynucleotides, polypeptides, and antibodies provided by this invention are expected to be particularly useful in diagnosis and treatment of breast cancer. Also provided is a general method for obtaining cDNA with similar properties that may be associated with breast cancer and other malignancies.