Our laboratory is involved in identifying genes that can be used as early diagnostic or prognostic markers in breast cancer. We previously identified a gene (NES1) that is expressed in normal but not in transformed mammary epithelial cells (MECs). NES1 is located on chromosome 19q13.4 within the kallikrein locus and thus was designated as human kallikrein 10 (hK10), although we have been unable to detect any protease activity. Importantly, hK10 expression is decreased in a majority of breast cancer cell lines. Transfection of hK10 into hK10-negative breast cancer cells reduces the tumorigenicity. Using methylation-specific PCR and subsequent sequencing, we demonstrate a strong correlation between hypermethylation of hK10 and loss of mRNA expression. Further analysis showed that essentially 100% of normal breast specimens had hK10 expression, whereas 46% of ductal carcinoma in situ (DCIS) and the majority of infiltrating ductal carcinoma (IDC) samples lacked the hK10 mRNA. Importantly, hK10-negative DCIS diagnosed at the time of biopsy were subsequently diagnosed as IDC at the time of definitive surgery. It has been shown that hK10 protein expression is regulated by steroids. In addition to breast cancers, hK10 is downregulated in cervical cancer, prostate cancer and acute lymphocytic leukemia, whereas it is upregulated in ovarian cancers. These results point to the paradoxical role of hK10 in human cancers and underscore the importance of further studies of this kallikrein.