N-(2-Hydroxyethyl)maleimide (HEM) is a bifunctional chemical compound featuring a reactive maleimide group and a terminal hydroxyl group. The maleimide moiety is highly electrophilic and undergoes rapid, selective Michael addition reactions with thiol groups (e.g., cysteine residues in proteins) under mild physiological conditions to form stable thioether bonds. The hydroxymethyl group provides a handle for further chemical modification, such as esterification or coupling to other functional molecules. Its primary application is in the field of bioconjugation and life sciences. HEM is widely used as a versatile linker or spacer for labeling proteins, peptides, antibodies, and other biomolecules with fluorescent dyes, biotin, drugs, or other probes. It is a key reagent for preparing maleimide-activated derivatives of fluorescent dyes (e.g., fluorescein-maleimide, Cyanine dye-maleimide) and for constructing antibody-drug conjugates (ADCs) or protein-polymer conjugates. The hydroxyl group allows for the introduction of solubility-enhancing or cleavable elements into the final conjugate. As a heterocyclic building block, it is also employed in organic synthesis to introduce the maleimide functionality into more complex molecular architectures, particularly in medicinal chemistry for the synthesis of potential bioactive compounds. It requires careful handling as it may cause skin and eye irritation.