Further research found that circRNA_ACAP2 specifically bound to miR-21-5p and inhibited its function, thereby regulating the direct target gene STAT3 of miR-21-5p, which proved that circRNA_ACAP2 functioned as a tumor suppressor gene in HNSC and was adjusted by miR-21-5p/STAT3 signaling axis regulation. The gene discussed is STAT3; the disease is neoplasm.