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OBJECTIVE Poncirus trifoliata (P. trifoliata) fruits exert phytotherapeutic effects, depending on their maturity level. However, the mechanism by which these phytotherapeutic effects are exerted remains undefined - especially in cancers. Therefore, in this study, we investigated the effects of the immature fruit extract of P. trifoliata on a B16 melanoma cell line. MATERIALS AND METHODS The effect of immature P. trifoliata extract on B16 cells was evaluated by MTT assay, cell proliferation, FACScan analysis of cell cycles, confocal imaging analysis, nuclear (Hoechst) staining, apoptosis assay (Annexin V-fluorescein isothiocyanate/propidium iodide staining), and Western blot assay. The capacity of immature P. trifoliata extract to inhibit the invasion and migration of B16 cells was assessed using the scratch-wound assay and Matrigel migration assay. The effect of immature P. trifoliata extract on mitochondrial function was determined via the mitochondrial membrane potential assay, activity, and fraction and cytosol proteins. RESULTS Treating B16 cells with a methanol extract of immature P. trifoliata (MEPT) significantly inhibited cell viability, migration, and invasiveness in a dose- (p<0.01) and time (p<0.01)- dependent manner. MEPT arrested the cells in the G1 phase of the cell cycle and led to the activation of the PI3K/AKT/p21 pathway. Furthermore, MEPT dose-dependently induced apoptosis in B16 cells by increasing the expression of the pro-apoptotic proteins Bax and Apaf-1, while decreasing the expression of the anti-apoptotic protein, Bcl-2. MEPT treatment also decreased mitochondrial membrane potential. CONCLUSIONS Immature P. trifoliata extract inhibited the growth of melanoma cells by inducing cell apoptosis through mitochondrial pathways. Therefore, further research into immature P. trifoliata extract as a potential therapeutic compound for melanoma treatment is warranted.