For breast cancer patients who received systemic therapy for advanced disease, only three studies are available relating the levels of serine proteases, that is, uPA (Foekens et al, 1995), human kallikrein 3 (PSA) (Foekens et al, 1999), or human kallikrein 10 (Luo et al, 2002) to a poor efficacy of first-line tamoxifen treatment. Here, KLK3 is linked to breast carcinoma.