Chemical probes comprising a ligand moiety, a reactive group (e.g. epoxide, haloacetyl or photoreactive group) and a tag unit (e.g. fluorophore or radioisotope) are widely used in affinity labeling to identify the target proteins of bioactive molecules. However, design and synthesis of highly functionalized chemical probes are often time-consuming. In this paper, we propose a simple design strategy for chemical probes bearing a small 2,3-dichloromaleimide (diCMI) unit, which serves as a combined reactive group and tag unit by reacting with a nucleophilic lysine residue near the ligand-binding site of the target protein to generate the 2-amino-3-chloromaleimide fluorophore. Model ligand–protein experiments confirmed that the diCMI unit has suitable reactivity and fluorogenic capability for efficient affinity labeling.