DKK1 and prostate cancer: Similar to other common cancer types, WNT signaling is continuously active in prostate cancer due to a downregulation of inhibitory WNT regulator genes, such as Wnt Inhibitory Factor 1 (WIF1), Dickkopf proteins (DKK1, DKK2, DKK3), and Secreted Frizzled Related Proteins (SFRP1, SFRP2, SFRP3, SFRP4), usually caused by methylation of the respective promoter [15,16].