Studies that delineate the impact of disease-associated enhancer variants (Table 2) reveal the relatively modest effect of enhancer variants on gene expression. The effect of enhancer variants has also been evaluated with massively parallel reporter assays in which the impact of mutations in enhancer sequences is determined through heterologous barcoding and high-throughput sequencing (reviewed in [75]). These high-throughput assays show that most variants that impact transcription induce 1.3- to 2-fold differences in target gene expression [73,74]. These findings align with the notion that enhancers modulate or fine-tune gene expression, analogous to a rheostat. Despite their modest transcriptional effects, enhancer variants can have large effects on downstream phenotypes. As an example, we highlight a SNP (rs12821256) associated with blond hair color in Europeans. This SNP lies in an enhancer that drives KITLG expression in developing hair follicles [33]. The blond-hair-associated SNP was shown to reduce enhancer activity by only 22% in vitro. Nonetheless, when the blond hair and ancestral alleles were evaluated in transgenic mice, the reduction in enhancer activity associated with the blond hair allele was sufficient to yield mice
