The size of ExAC also makes it possible to directly observe mutational recurrence: instances in which the same mutation has occurred multiple times independently throughout the history of the sequenced populations. For instance, among synonymous variants, a class of variation expected to have undergone minimal selection, 43% of validated de novo events identified in external datasets of 1,756 parent-offspring trios8,9 are also observed independently in our dataset (Figure 2a), indicating a separate origin for the same variant within the demographic history of the two samples. This proportion is much higher for transition variants at CpG sites, well established to be the most highly mutable sites in the human genome10: 87% of previously reported de novo CpG transitions at synonymous sites are observed in ExAC, indicating that our sample sizes are beginning to approach saturation of this class of variation. This saturation is detectable by a change in the discovery rate at subsets of the ExAC data set, beginning at around 20,000 individuals (Figure 2b), indicating that ExAC is the first human exome-wide dataset large enough for this effect to be directly observed.
