We next examined the impact of MCL1 si-NPs in parental ERα+ breast cancer treated with fulvestrant, first verifying Mcl-1 knockdown in parental HCC1428, MCF7, and T47D cells at 48 h post-treatment, revealing 70–90% fewer MCL1 transcripts as compared to cells treated with LUC si-NPs (Fig. 5a). Here, MCL1 is linked to breast cancer.