Although associations between certain gene variants and ND risk have been consistently replicated (such as de novo disruptive mutations in CHD8, ADNP and DYRK1A [19] among ASD and ID simplex families), hundreds of ND risk genes remain undiscovered or have not been associated with NDs with sufficient statistical significance owing to ultra-low mutation frequencies in the patient population. This evidence concerns the gene CHD8 and Norrie disease.