The Norrin cystine knot growth factor (NDP) gene is a 24,615-nt long gene located on chromosome Xp11.3, contains 3 exons (Fig. 1), and is widely expressed in several tissues, including the eye, ear, and brain.[4] The NDP-encoded secretory protein Norrin consists of 133 amino acids, which mediates the activation of the Wnt/β-catenin pathway, thereby regulating cell division and differentiation, and plays a crucial role in the development of the retina, cochlea, and central nervous system.[5–7] The variants in NDP have been found to be responsible for ND. This evidence concerns the gene NDP and Norrie disease.