With respect to breast cancer in particular, several kallikrein family members, namely KLK3 (Yu et al, 1996), KLK5 (Yousef et al, 2003c), KLK6 (Anisowicz et al, 1996), KLK10 (Liu et al, 1996; Dhar et al, 2001), KLK12 (Yousef et al, 2000b), KLK13 (Yousef et al, 2000a), and KLK14 (Borgono et al, 2003) were shown to be differentially expressed at the mRNA or protein levels within the cancerous tissues or serum of such patients. Here, KLK4 is linked to breast cancer.