Secreted frizzled-related protein 1, also known as sFRP1, is a 35 kDa prototypical member of the SFRP family. SFRP family consists of five secreted glycoproteins in humans acting as extracellular signaling ligands. Each is approximately 3 amino acids in length and contains a cysteine-rich domain (CRD) that shares 3-5% sequence homology with the CRD of Frizzled (Fz) receptors, a putative signal sequence, and a conserved hydrophilic carboxy-terminal domain. SFRPs act as soluble modulators of Wnt signaling, counteracting Wnt-induced effects at high concentrations and promoting them at lower concentrations. SFRPs are able to bind Wnt proteins and Fz receptors in the extracellular compartment. The interaction between SFRPs and Wnt proteins prevents the latter from binding the Fz receptors. The Wnt pathway plays a key role in embryonic development, cell differentiation and cell proliferation. The deregulation of this critical developmental pathway occurs in several human tumor entities. Mouse sFRP1 is highly expressed in kidney and embryonic heart, as well as in the eye, where it is principally localized to the ciliary body and the lens epithelium.

FRZB also known as sFRP-3, is a secreted protein containing a domain similar to the putative Wnt-binding region of the frizzled family of transmembrane receptors. FRZB is widely expressed in adult mammalian tissues. In the Xenopus gastrula, FRZB is regulated as a typical Spemann organizer component. FRZB also functions as a competitor for the cell-surface G-protein receptor Frizzled. It is espically important in embryonic development. Defects in FRZB gene can cause female-specific osteoarthritis (OA) susceptibility. FRZB may serve an important role in determining hip shape and may modify the relationship between hip shape and OA.

The Secreted frizzled-related protein (SFRP) family consists of five secreted glycoproteins in humans (SFRP1~5) that act as extracellular signaling ligands. Each SFRP is approximately 3 amino acids in length and contains a cysteine-rich domain (CRD) that shares 3-5% sequence homology with the CRD of Frizzled (Fz) receptors, a putative signal sequence, and a conserved hydrophilic carboxy-terminal domain. SFRPs are able to bind Wnt proteins and Fz receptors in the extracellular compartment. The interaction between SFRPs and Wnt proteins prevents the latter from binding the Fz receptors. The Wnt pathway plays a key role in embryonic development, cell differentiation and cell proliferation. sFRP2 is a member of the SFRP family acting as soluble modulators of Wnt signaling and contains a cysteine-rich domain homologous to the putative Wnt-binding site of Frizzled proteins called FZ domain and a NTR domain.sFRP2 inhibites hypoxia induced endothelial cell apoptosis and increases endothelial cell migration. It prevents mesoderm specification and maintains the cells in the undifferentiated state. SFRP2 is also a novel stimulator of angiogenesis that stimulates angiogenesis via a calcineurin/NFAT pathway, thus is regarded as a favorable target for the inhibition of angiogenesis in solid tumors. Mouse sFRP2 is highly expressed in the eye and is also detected in heart and lung at low level.