The nucleophilic aromatic substitution reaction required for labeling can be tuned by the aromatic substitution pattern.[13] In this perspective, the irreversible MIF and MIF2 inhibitor 4‐iodo‐6‐phenylpyrimidine (4‐IPP) holds promise for development of probes for covalent modification of the nucleophilic N‐terminal proline residue.[14] 4‐IPP proved to be a promising inhibitor since treatment with 4‐IPP exhibited inhibition of cancer growth and osteoclast formation in cell‐based studies and animal models.[15] Nevertheless, target engagement of 4‐IPP in the proteome remains unknown. The gene discussed is MIF; the disease is cancer.