We first described a novel high affinity and selective PACE4 inhibitor, namely the ML-peptide, (Ac-LLLLRVKR-NH2) [21], which is 20 fold more potent for PACE4 than furin and displays anti-proliferative effects in the prostate cancer cell lines DU145 and LNCaP [21]. This evidence concerns the gene PCSK6 and prostate cancer.