In order to understand the normal function of Wnt signaling in mammary glands, we chose to study how loss of function of Wnt signaling affected mammary development. There are many Wnt-dependent signaling events, but only one pathway has so far been associated with the stem cell functions and oncogenic properties. This so-called canonical pathway is mediated by the interaction of Wnt ligands with a pair of cell surface receptors, comprising a Frizzled receptor and an Lrp5 or-6 receptor. Binding of the Wnt ligand to the Frizzled and Lrp5/6 receptor is followed by the recruitment of axin from the b-catenin destruction complex, stabilization of b-catenin and, transactivation of specific target genes via a b-catenin/TCF complex. There are many members of the Wnt family of secreted lipoglycoprotein ligands,CX-4945 and several are expressed during mammary gland development. Similarly, there are 10 known Frizzled homo-logues, of which Frizzled 1–8 are known to be expressed in mammary epithelial cells. However, there is an absolute requirement for either Lrp5 or Lrp6 for canonical Wnt signaling. Lrp5 and-6 belong to the LDL receptor related protein family of single-span transmembrane receptors, which mediate binding and internalization of various lipoprotein particles. Mammary development fails in the absence of Lrp6; both epithelial outgrowth of the placode and the formation of the host adipose tissue is affected. The role of Lrp6 in adult tissues is unclear,CYT387 but loss of function mutations have been linked with human cases of coronary artery disease. In contrast, Lrp5 null mice are viable, although they exhibit defects in bone ossification and vascularization of the eye. In adult tissues, Lrp5 mRNA and protein levels are high and widely expressed in tissues such as bone, pancreas, central nervous system, and in phagocytic cells. Loss of function mutations have been associated with heritable cases of osteoporosis as well as Type I diabetes. In the mammary gland, Wnt signaling is required for specification and outgrowth of the mammary rudiment from the embryonic skin, and a Wnt reporter strain shows high Wnt signaling activity at this stage.