Wnt receptors. Several cell surface proteins directly bind Wnt proteins, including Frizzled (FZD), LRP5 & 6 (fly homolog is Arrow), PTK7 (fly homolog is Otk), ROR1 & 2, and RYK (fly homolog is Drl). Abbreviations: aa = amino acids, CRD = cysteine-rich domain, C = carboxy terminus, Ig = immunoglobulin domain, N = amino terminus, WIF = Wnt inhibitory factor