In this cancer, most of the KLKs, except KLK4 and KLK15, show reduced mRNA and/or protein expression levels compared to expression of the KLKs in normal breast tissue.9,23,66,70,71KLK3, 8 and 11 mRNA expression is not changed in malignant breast tissue compared to normal breast tissue; KLK1, 2, and 5–12 mRNA expression is decreased; KLK4 and 15 are increased. Here, KLK4 is linked to cancer.