CG0009 is a potent and highly selective glycogen synthase kinase 3 (GSK3) inhibitor that inhibitions proliferation, induces apoptosis, and activates the p53-Bax pathway in breast cancer cells through cyclin D1 depletion. CG0009 inhibit breast cancer cell growth through cyclin D1 depletion and p53 activation, and thus may provide an innovative treatment approach for breast cancer resistant to hormone-based therapies.