ABCA4 and severe early-childhood-onset retinal dystrophy: Over 1000 disease-causing variants in ABCA4 have been identified to date, with the resulting phenotype varying between COD, CORD and Stargardt disease (STGD).66 In general, biallelic null variants are more commonly associated with severe and earlier onset CORD and childhood-onset STGD,67 68 whereas biallelic missense variants are associated with milder disease such as later onset and foveal-sparing forms of STGD.69 70 Functional outcome is dependent on both the variant itself and interaction with other ABCA4 variants (and other genetic modifiers).