Propargyl-PEG1-acid, a PEG-based PROTAC linker, enables the synthesis of BTK-CRBN PROTACs, specifically Ibrutinib-based PROTAC 4 and PROTAC 5. At a concentration of 10 μM, PROTAC 5 facilitates the degradation of BTK and induces the degradation of CSK, LYN, and LAT2[1].