The degree of ABCA4 inactivation relates to the ABCA4 disease spectrum, with STGD associated with milder sequence variants and thereby often milder inactivation of ABCA4 and a milder phenotype, compared with severe variants, resulting in he complete absence of ABCA4 function and thereby more severe disease, such as cone and rod dystrophy. Here, ABCA4 is linked to cone-rod dystrophy.