Members of the dickkopf-related protein family (Dkk-1, -2, -3, and -4) are secreted proteins with two cysteine-rich domains separated by a linker region. Dkk-3 and -4 also have one prokineticin domain. Dkk-1, -2, and -4 function as antagonists of canonical Wnt signaling by binding to LRP5/6, preventing interaction of LRP5/6 with Wnt-Frizzled complexes. Dkk-1, -2, and -4 also bind cell surface Kremen-1 or -2 and promote the internalization of LRP5/6. Antagonistic activity of Dkk-3 has not been demonstrated. Dkk proteins have distinct patterns of expression in adult and embryonic tissues and have a wide range of effects on tissue development and morphogenesis. The Dkk family also includes Soggy, which is homologous to Dkk-3 but not to the other family members. Soggy has not been shown to inhibit Wnt signaling, and its role in the pathway is unclear.