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. The gene discussed is ABCA4; the disease is Cone rod dystrophy.