FZD4 and Norrie disease: In congenital Norrie disease, loss of the non-WNT FZ4 ligand norrin impairs retinal vascularisation and thereby causes blindness; norrin activates the beta-catenin pathway through its receptor FZ4 and LRP5, akin to the canonical WNT signalling pathway (Fig. 2) (Xu et al., 2004).