ESR1 and breast cancer: Therefore, we resynthesized the MHI-148 palbociclib conjugate, hereafter referred to as MHI-palbociclib, as reported in a patent (US 2019/0343958), to examine its effect and explore the mechanism of action of the conjugate in estrogen receptor-positive MCF-7 and triple-negative MDA-MB-231 human breast cancer cell lines.