The most severe problems are caused by chromosomal deletions in the region of the NDP gene, causing the prevention of the gene product, or even that of the neighboring MAO genes. When the mutations simply change a single amino acid in norrin, the effects are less widespread and severe. However, the location and type of the NDP mutation does not necessarily determine the degree of severity of the disease, since highly varying clinical signs have been diagnosed in patients carrying exactly the same mutation.