eLife Assessment
This manuscript presents a valuable methodological approach for investigating context-dependent activity of cis-regulatory elements within defined genomic loci. The authors combine a locus-specific massively parallel reporter assay, enabling unbiased and high-coverage profiling of enhancer activity across large genomic regions, with a degenerate reporter assay to identify nucleotides critical for enhancer function. The data supporting the conclusions are solid, highlighted by the successful identification and characterization of both previously known and new regulatory elements across multiple developmental stages, cell types, and species; however, concerns regarding assay sensitivity, statistical rigor in distinguishing active regions, and limitations inherent to the design of the reporter assays remain to be addressed. With strengthened quantitative analysis, statistical validation, and additional functional experiments to directly establish regulatory element-gene relationships, this study will be of broad interest to researchers investigating gene regulation mechanisms in development and disease.