ABCA4 and severe early-childhood-onset retinal dystrophy: The exact sequence of the disease process for STGD is not completely understood, however the generally accepted hypothesis suggests that defective transport of vitamin A derivatives due to mutant ABCA4 protein results in abnormal accumulation of visual cycle by-products (bis-retinoids, lipofuscin) in the retinal pigment epithelium (RPE) [7] with consequent RPE degeneration and photoreceptor disruption [8–11].