CXCR4 and breast cancer: Mechanistically, miR-139 attenuates CXCR4 signaling to inhibit breast cancer invasiveness via inactivation of phosphorylated-Akt (pAktSer473), rather than phosphorylated-ERK signaling, in the miR-139-transduced MDA-MB-231 breast cancer cells (Figure 3C,D).