ABCA4 and Alzheimer disease: Stargardt disease can be distinguished in three different forms: (I) STGD1, the most common form, displays an AR homozygous or compound heterozygous transmission and is caused by mutations in the ABCA4 gene; (II) STGD3, which is determined by AD mutations in the ELOVL4 gene; (III) STGD4, a rare AD form associated with mutations in the PROM1 gene.