When ABCA4 is defective, N-ret-PE irreversibly forms a toxic, insoluble, bisretinoid known as A2PE which is deposited in retinal pigment epithelium (RPE) cells during the process of disc shedding and phagocytosis, eventually leading to cell death and macular degeneration [33]. The gene discussed is ABCA4; the disease is macular degeneration.