To explore whether ZLD1039 treatment has a similar effect on cell viability as EZH2 knockdown, the in vitro antiproliferative activities of ZLD1039 against various breast cancer cell lines were examined using the 3–(4,5-dimethylthiazol–2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. The gene discussed is EZH2; the disease is breast cancer.