PTEN and oral cavity squamous cell carcinoma: Similarly, a hypoxic tumor microenvironment can induce oral squamous cell carcinoma cells (OSCCs) to generate more miR-21-containing exosomes that are then transferred to normoxic OSCCs to enhance their migration and invasion by inhibiting expression of phosphatase and tensin homolog (PTEN) and PDCD4 (Li et al., 2016).