A) Schematic illustrating GADD45A 250 nt enhancer mutagenesis screen using the STARRSeq system (hSTARRSeq_ORI). Substitutions and deletions of every nucleotide position are depicted as ‘X’; p53RE in blue; open reading frame as ‘ORF’; polyadenylation site as ‘polyA’. B) Heatmap representing expression mediated by each enhancer variant relative to the wild-type enhancer from the same cell line and treatment condition. Relative position in the enhancer (1–250 nt) is indicated on the x-axis. Each deletion (‘Del’) or base substitution (‘A’, ‘C’, ‘G’, ‘T’) is indicated as a row label. ‘Grey’ color in row ‘Del’ indicates a redundant position when >1 consecutive base is identical. Relevant motifs discussed in the text are highlighted (red dashed line) including: GADD45A native motif sequence, relative position, name (‘p53RE’, ‘AP1’) and PWM logos (JASPAR 2022 (Castro-Mondragon et al., 2022)) are in panel C. Cell lines and treatment conditions are indicated above each heatmap.