Given that ENCODE did not assay all cell types, or all TFs, and in particular has sampled few specialized or developmentally restricted cell lineages, these proportions must be underestimates of the total amount of functional bases. However, many assays were performed on more than one cell type, allowing assessment of the rate of discovery of new elements. For both DHSs and CTCF sites, the number of new elements initially increases rapidly with a steep gradient for the saturation curve and then slows with increasing numbers of cell types (Supplementary Figure R1 and R2). With the current data, at the flattest part of the saturation curve, each new cell type adds on average 9,500 DHS elements (across 106 cell types) and 500 CTCF-binding elements (across 49 cell types), representing 0.45% of the total element number. We modelled saturation for the DHSs and CTCF-binding sites using a Weibull distribution (r2 > 0.999) and predict saturation at approximately 4.1 million (S.E. = 108,000) and 185,100 (S.E. = 18,020) sites, respectively, suggesting that we have discovered around half of the estimated total DHSs. These
