ABCA4 and glycogen storage disease VI: We have started to determine someof the cis-modifiers (see the example of thep.[Gly863Ala,Gly863del] allele above), but most of these remain obscure.Trans-modifiers will be even harder to identify, but with thecomplete sequencing of the ABCA4 locus (Bauwens et al., 2019; Sangermano et al., 2019; Zernant etal., 2014b) and the entire exome and genome, these are likely to be foundthrough the analysis of very large familial cohorts, which are, fortunately,possible to obtain even for a relatively rare Mendelian disease.