Abstract: The present invention relates to a novel gene, Di12, that is differentially expressed as a 1.35 kb RNA in breast cancer tissues and cell lines, and in several normal tissues. The full length cDNA encodes a protein of 339 amino acids. Antibodies to the gene product were developed to investigate the expression of Di12 in breast cancer cell-lines and tumors. The Di12 protein was found in tissue sections of infiltrating ductal carcinomas (IDCs), but not in benign or normal breast specimens. Di12 wag also present in IDC-breast cancer patient sera, and its expression level increased markedly if IDC was accompanied by lymph node or distal metastases. As IDC constitutes ˜70% of breast cancers seen clinically, the level of Di12 expression is useful for diseases diagnosis predicting disease progression and monitoring a therapeutic treatment.