ERD-308 (ERD308) is a highly potent PROTAC degrader of estrogen receptor (ER) with the potential for treating ER positive breast cancer treatment (DC50: 0.17 nM (ER in MCF-7 cells)). ERD-308 induces >95% of ER degradation at concentrations as low as 5 nM in both cell lines (DC50 (concentration causing 50% of protein degradation) of 0.17 nM and 0.43 nM in MCF-7 and T47D ER+ cells, respectively). ERD-308 also induces more complete ER degradation than fulvestrant, the only approved selective ER degrader (SERD), and is more effective in inhibition of cell proliferation than fulvestrant in MCF-7 cells.