High FRA-1 levels are found in aggressive TNBC, whereas ER+ BC cell lines typically have low FRA-1 levels and are poorly invasive.14, 15 To examine whether MLK3 promotes FRA-1 expression in ER+ BC cells, we utilized MCF7 cells engineered to overexpress MLK3 upon treatment with the transcriptional inducer AP21967 (MCF7iMLK3).9, 22, 23 As shown in Figure 1a, induced MLK3 expression increases FRA-1 protein expression, and drives cell migration in both transwell9 and wound-healing assays (Supplementary Figure 1). This evidence concerns the gene FOSL1 and breast cancer.