ABCA4 and Alzheimer disease: The most prevalent IRD-causing genes across all populations are ABCA4 (MIM: 601691), RHO, USH2A (MIM: 608400) and RPGR (MIM: 312610), which account for high percentages of some of the IRD subtypes, i.e. 70–71% in STGD1/AR-MD/AR-CRD6, 19–25% of AD-RP, 10% of AR-RP, and 70% of XL-RP, respectively1.