Tumour biomarkers in current clinical use are generally proteins secreted by the tumour such as CA-125 (Rustin et al. 1996) and PSA (prostate specific antigen) (Kelly et al. 1993), or cell surface receptors that can predict response to specific targeted treatments, such as HER2 overexpression in breast cancer which is predictive of response to targeted inhibition of HER2 with trastuzumab (Baselga et al. 1996). Here, KLK3 is linked to breast cancer.