PRPF31 and retinitis pigmentosa 1: On the basis of these observations, the mechanism of incomplete penetrance in this form of RP has been described as ‘variant haploinsufficiency’, in which the absence of a second wild-type PRPF31 allele is sometimes sufficient to produce disease, and sometimes is not, depending on the nature of the mutant allele inherited and the nature of the wild-type allele inherited.