ABCA4 and Rod-cone dystrophy: 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.