NTR domain, Secreted frizzled-related protein (Sfrp) 3-like subfamily; composed of proteins similar to human Sfrp3 and Sfrp4. Sfrps are soluble proteins containing an NTR domain C-terminal to a cysteine-rich Frizzled domain. They show diverse functions and are thought to work in Wnt signaling indirectly, as modulators or antagonists by binding Wnt ligands, and directly, via the Wnt receptor, Frizzled. They participate in regulating the patterning along the anteroposterior axis in vertebrates. Human Sfrp3 may suppress the growth and invasiveness of androgen-independent prostate cancer cells.