Fig. 2. Ultraconserved enhancers are mostly robust to mutagenesis.

a, Schematic of the possible functional outcomes of enhancer mutagenesis. b, Distribution of 23 ultraconserved enhancers among the three bins of enhancer mutagenesis outcomes. Representative embryo images are shown for each tested allele of three example enhancers (boxes in shades of green). In one case (hs111, bottom right), residual enhancer activity was still observed after mutating 20% of ultraconserved base pairs. Data for all 23 enhancers are provided in Extended Data Figure 3a, with P values in Supplementary Table 1. Examples of scoring guidelines are described in Extended Data Figure 2, with scoring data for all 23 enhancers in Supplementary Figure 1. e11.5 embryos have a crown-rump length of 6 mm on average. * indicates a loss of LacZ staining in the mutant allele, FB – forebrain, HB – hindbrain.