Human kallikrein 6 (protease M/zyme/neurosin) was originally identified based on its aberrant expression in tumor cells and is considered a biomarker for ovarian cancer. Here, we describe the identification, cloning, and tissue expression of three novel transcript variants of the KLK6 gene that encode for wild-type kallikrein 6. Contrary to the classical form, transcript variants contain one untranslated exon, exploit intronic sequences, and are likely products of alternative promoters. In addition, we cloned splice variants 2 and 3 produced by splicing out exons 3 and 4, respectively. Given the potential diagnostic applications of kallikrein 6 at both the mRNA and protein levels, we developed a duplex RT-PCR, in order to differentially detect and quantitate mRNA species corresponding to splice variants. We show that in normal mammary epithelial cells and mammary tumor cell lines that overexpress the KLK6 gene, splice variants account for approximately 10-20% of all mRNA species. (C) 2004 Elsevier Inc. All rights reserved.