Regulator of cell surface receptor signal transduction. Plays a central role in retinal vascularization by regulating norrin (NDP) signal transduction. Acts in concert with norrin (NDP) to promote FZD4 multimerization and subsequent activation of FZD4, leading to promote accumulation of beta-catenin (CTNNB1) and stimulate LEF/TCF-mediated transcriptional programs. Suprisingly, it only activate the norrin (NDP)-dependent activation of FZD4, while it does not activate the Wnt-dependent activation of FZD4, suggesting the existence of a Wnt-independent signaling that also promote accumulation the beta-catenin (CTNNB1) (By similarity). Component of a complex, at least composed of TSPAN12, FZD4 and norrin (NDP). Palmitoylated; required for interaction with ADAM10. The precise position of palmitoylated residues is unclear and occurs either on Cys-9, Cys-12 and/or Cys-83 (By similarity). Belongs to the tetraspanin (TM4SF) family.