Breast cancer cells also secrete activin A (a member of transforming growth factor (TGF)-β), noggin (a bone morphogenetic protein (BMP) antagonist) and dikkopf-1 (DKK-1; a wingless (Wnt) protein antagonist), all of them inhibiting osteoblast differentiation [5,7]. Here, DKK1 is linked to breast carcinoma.