Abstract A ligation strategy based on the Diels–Alder [4+ 2] cycloaddition for the two-step activity-based labeling of endogenously expressed enzymes in complex biological samples has been developed. A panel of four diene-derivatized proteasome probes was synthesized, along with a dienophile-functionalized BODIPY (TMR) tag. These probes were applied in a Diels–Alder labeling procedure that enabled us to label active proteasome β-subunits ...