Human DKK-1 (Dickkopf related protein 1) is a secreted antagonist of Wnt/beta-Catenin signaling. It interacts with LRP-6, and internalized complexes of DKK-1/LRP-6/Kremen 2 can downregulate Wnt signaling. The balance between Wnt signaling and Human DKK-1 inhibition is critical for bone formation and homeostasis. In addition, Human DKK-1 can act as either a tumor suppressor or promoter of metastasis.