ESR1 and breast cancer: As shown in Figure 3A, a series of 5′ and 3′ progressive deletions were generated by nested PCR using MLK3-P1 as a template, and their activities were measured by their ability to drive luciferase expression in MCF7 (ER+) and MDA-MB-231 (ER−) breast cancer cell lines (Figure 3B and 3C).