Analysis of the effects of transfecting MDA-MB-231 (Figure 2B) and MCF-7 (Figure 2C) breast cancer cells with isomiR-31s showed a greater repression of STK40 mRNA expression by miR-31-H and miR-31-M to about 22% and 50% of control for MDA-MB-231 and MCF-7 cells, respectively, than by miR-31-P (to 75% and 95% of control in MDA-MB-231 and MCF-7 cells, respectively). This evidence concerns the gene STK40 and breast cancer.