Our review has revealed variations in ten genes that relate to ID/GDD including CACNA1A, CACNA1C, CACNA1I, CACNA1H, CACNA1D, CACNA2D1, CACNA2D2, CACNA1E, CACNA1F, and CACNA1G. Most variants exhibited gain-of-function effect. This evidence concerns the gene CACNA1D and Global developmental delay.