ABCA4 and severe early-childhood-onset retinal dystrophy: In addition to the c.4253+43G>A sequence alteration [18,19,20,40], the two deep intronic variants c.4539+1770C>A and c.5196+1015A>G [19] might further help to explain the missing heritability in STGD-affected patients lacking clear mutations in ABCA4 exons.