ESR1 and breast cancer: In MCF7 cells, L1 transposons were found to be bound by a total of 99 TFs, including ESR1, MYC, FOXA1 and E2F1, which have well-established and critical roles in cell growth and division and have been linked to breast cancer development [60,61,62,63,64,65,66,67] (Figure 2).