ESR1 and breast cancer: Analysis of BRD4 density by ChIPseq was used to identify SEs and their classification based on their location in the genome (enhancer, promoter, gene body intron, gene body exon, 3 prime, or other, see “Methods”) in an unbiased fashion in two HER2+ breast cancer cell lines, SKBR-3 (ER−/PR−) and BT474.m1 (ER+/PR+) (Fig. 1b, Supplementary Fig. 1a and Supplementary Data 1 and 2).