KLK6 and neoplasm: To delineate the molecular mechanisms underlying these concentration‐dependent effects of KLK6 on the malignant phenotype, we engineered MDA‐MB‐231 transfectant clones that stably express increasing levels of KLK6, including a clone that expresses intermediate levels (between normal and overexpression), and studied primary tumor growth following injection of these clones in SCID mice and their lung metastatic potential.