The natural killing (NK) cells can directly kill cancer cells, and curcumin can enhance the cytotoxicity effect of NK cells when NK cells are co-cultured with human breast cancer MDA-MB-231 cells, which is highly associated with signal transducer and activator of transcription 4 (STAT4) and signal transducer and activator of transcription 5 (STAT5) activation [52]. Here, STAT4 is linked to breast carcinoma.