ABCA4 and Stargardt disease: A large amount of allelic heterogeneity has been reported in the ABCA4 gene; over 800 different mutations were mapped on the ABCA4 gene in Stargardt disease, which causes various phenotypes and multiple severities of the disease.[43,44,45] The age of onset is a prognostic factor; patients with lower age of onset (early-onset) will experience more severe disease.[8]