Many human genetic diseases by their very nature would only be expected to show subtle effects on cellular behaviour, since those individuals show essentially normal differentiation, development and cellular function and only present symptoms of disease after birth, in old age, or upon exposure to environmental triggers. This alongside an inherent variability in both the iPSC derivation process and differentiation into specific cell types makes it necessary to perform comparisons of many independently derived cell lines from multiple healthy and diseased individuals in order to detect such subtle changes. This can be ameliorated by genetic engineering to introduce or repair putative causative alleles to generate isogenic cell line pairs that have identical genetic backgrounds, and differ in only a single genetic change (Fig. 2). This allows detection of subtle phenotypes that would otherwise be masked by variations in cellular phenotype due to the different genetic backgrounds of the donors.
