ABCA4 and retinitis pigmentosa: As proposed by Maugeri et al. [16], two null ABCA4 alleles such as homozygous mutation IVS30+1G>T results in retinitis pigmentosa; compound heterozygosity for a null and a moderately severe mutation such as IVS30+1G>T and IVS40+5G>A causes cone-rod dystrophy; two moderately severe mutations or a mild and a severe allele such as IVS30+1G>T and 2588G>C (a mild mutation) cause Stargardt disease.