Discloses isolated bivalent antibodies or a bivalent fragment thereof to a low density lipoprotein-related protein 6 (LRP6) that potentiates a Wnt signal by clustering one or more LRP6 receptors in the presence of an unblocked LRP6 binding protein, and an isolated monovalent antibody or monovalent binding fragment thereof to a LRP6 that avoids potentiating a Wnt signal by clustering one or more LRP6 receptors in the presence of an unblocked LRP6 binding protein. Further discloses specific sequence structures of such antibodies and use in the treatment of cancer.
