Usnic acid has been previously shown to induce apoptosis of various cancer cell lines, such as human ovarian carcinoma A2780, human colon adenocarcinoma HT29, human breast adenocarcinoma MCF-7, human cervix adenocarcinoma HeLa, human promyelocytic leukemia, human T-cells lymphocyte leukemia, Jurkat, and human breast adenocarcinoma SK-BR-3 cells, at lethal doses through a caspase-dependent pathway by activating caspase-3 [5]–[10], [22], [23]. Here, CASP3 is linked to acute promyelocytic leukemia.