PIK3R1 and breast cancer: Regarding the fact that PDK1 facilitated tumor cell migration and infiltration into the lungs by regulating epithelial–mesenchymal transition (EMT) [49], as well as the fact that miR-155 harnessed the tumor growth of BC cells in vivo via PIK3 R1-PDK1/AKT-FOXO3a pathway [78], a study revealed that miR-181c boosted brain metastasis of BC cells via directly downregulating PDK1 to destroy the blood–brain barrier (shown in Figure 4) [69], which compelled us to hypothesize that PDK1 might take part in the organotropic metastasis of BC.