RPE65 and respiratory distress syndrome in premature infants: However, this does not seem to be the case in mice, because heterozygous KIs were either found to have approximately 50% the activity of WT [40,54] or, when compared to WT/KO heterozygotes, only a mild dominant effect over isomerase activity, which again reduced levels of 11-cis retinal to 50% WT values [53], which is a level in line with normal visual functioning in most RPE65-associated IRDs [18,19].