ABCA4 and severe early-childhood-onset retinal dystrophy: STGD1 probands display very different phenotypes based on the allele combinations, from early-onset STGD1 or panretinal cone-rod dystrophy because of two severe alleles, to late-onset STGD1 characterized by foveal sparing because of p.(Asn1868Ile) or p.(Gly1961Glu) in trans with a severe allele (7).