MTOR and neoplasm: To gain further mechanistic insights into the roles of miR‐21‐5p/PIK3R1 axis, we hypothesized that exosomal miR‐21‐5p derived from MSCs activates the PI3K/Akt/mTOR signalling pathway in OS, leading to OS proliferation and invasion, because the PI3K regulatory subunit p85α/PIK3R1 can exert a tumour‐suppressor effect by negatively regulating the PI3K‐Akt signalling pathway.28