DKK1 and hereditary clear cell renal cell carcinoma: DKK1 is a secreted protein that plays a crucial role as a negative regulator of the Wnt signaling pathway, and downregulation of DKK1 in colon cancer, breast cancer, hepatocellular carcinoma, and renal cell carcinoma suggests that DKK1 is an antagonist of the Wnt signaling pathway.