PIK3R1 and osteosarcoma: Bioinformatics analysis and dual‐luciferase reporter gene assays validated the targeted relationship between exosomal miR‐21‐5p and PIK3R1; we further demonstrated that miR‐21‐5p‐abundant exosomes derived human bone marrow MSCs could activate PI3K/Akt/mTOR pathway by suppressing PIK3R1 expression in osteosarcoma cells.