Membrane type serine protease 1 (MT-SP1) is a representative member of a large family of related enzymes known as type II transmembrane serine proteases or membrane type serine proteases. MT-SP1 has been implicated in the selective proteolysis of key extracellular substrates but its physiological role is still not fully understood. MT-SP1 expression at the protein and RNA level has been previously examined by non-quantitative methods such as in situ hybridization, Northern blotting and immunohistochemistry. To establish an introductory understanding of the quantitative mRNA expression of MT-SP1 and to correlate these levels with urokinase-type plasminogen activator receptor (uPAR), a key component of extracellular proteolysis, quantitative RT-PCR was carried out. RNA expression was analyzed in 34 human cancer cell lines, 26 human tissues and 18 primary human breast cancer tissue samples. MT-SP1 mRNA is highly expressed in many breast, ovarian, prostate and colon cancer cell lines and normal human tissues of endodermal origin. At the transcript level, MT-SP1 shows a highly statistically significant correlation (Pearson's product moment correlation r = 0.784, p < 0.001) with uPAR in human breast cancer tissue. The exact role of MT-SP1 in concert with proteins such as uPAR and other members of the plasminogen activator cascade has yet to be ascertained. However, the significant correlation between MT-SP1 and uPAR transcript levels in this initial study suggests further work to establish the role of MT-SP1 as a possible prognostic, diagnostic or therapeutic target for breast cancer.