RAPGEF5 and congenital stationary night blindness: Using these tools, successful identification of the genetic variants responsible for simple Mendelian traits have been identified, including a novel variant in glutamate metabotropic receptor 6 (GRM6) associated with congenital stationary night blindness [4] and a nonsense variant in rap guanine nucleotide exchange factor 5 (RAPGEF5) associated with equine familial isolated hypoparathyroidism [5].