In vitro experiments in MDA-MB-231 triple-negative breast cancer cells showed that α-mangostin and the ethanol extract significantly inhibit PD-L1 expression when treated for 72 h with 10 μM or 10 μg/mL, respectively, and partially inhibit glycosylation of PD-L1 when compared to untreated controls. The gene discussed is CD274; the disease is triple-negative breast carcinoma.