To elucidate the mechanism underlying the anticancer activity of S-023-0996, global proteomic analysis was performed in human breast cancer cells. Mass spectrometry-based proteomic profiling was carried out in MDA-MB-231 cells following treatment with four experimental groups: Tazemetostat (Taz), Gefitinib (Gef), a combination of Tazemetostat and Gefitinib (Taz + Gef), and S-023-0996 (41a), along with a vehicle control (Control). The proteomic profiles of each treatment group were compared with that of the control, and the differential protein expression specific to S-023-0996 was subsequently analysed to elucidate the molecular mechanisms underlying its superior anticancer activity.