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. This evidence concerns the gene KLK3 and breast cancer.