ADRM1 and plasma cell myeloma: Here, the authors identify and characterize XL5, a lead compound that binds to the N-terminal Pru domain of human Rpn13 (hRpn13), solve the NMR structure of XL5-ligated hRpn13 Pru and develop XL5-PROTACs that preferentially target an identified hRpn13 Pru fragment present in multiple myeloma cells.