These findings, however, do not rule out the possibility that the abnormal human ERG derives from a hybrid photoreceptor cell type that also expresses S-opsin. It is possible that there are gene regulatory differences between mice and humans such that in human NR2E3 mutants, S-opsin shows a type I pattern of derepression rather than a type II as in seen in the rd7 mouse, and is therefore expressed in all of the hybrid photoreceptor cells. Alternatively, the ratio of supernumerary S-cones to hybrid photoreceptors produced in the retina of ESCS patients might be such that a higher percentage of the presumptive rods in ESCS patients become S-cones rather than hybrid photoreceptors. As discussed above, this ratio could depend either on the relative percentages of two distinct rod precursor populations or on stochastic effects on regulatory gene expression.