6-Biotinamidohexanoic acid (also known as Biotin-X, SE or Biotin-XX, SE) is a derivative of biotin (vitamin H or B7) featuring a 6-aminohexanoic acid spacer arm. This extended linker increases the distance between the biotin moiety and the target molecule, significantly reducing steric hindrance during subsequent binding with avidin or streptavidin. The compound is typically supplied as an N-hydroxysuccinimide (NHS) ester, which is highly reactive towards primary amines, enabling efficient and stable biotinylation of proteins, antibodies, peptides, and other amine-containing biomolecules. Its primary application is as a versatile biotinylation reagent in life science research and diagnostics. The biotin tag allows for the detection, purification, or immobilization of the labeled molecule using the extremely high-affinity interaction with streptavidin. This is crucial in techniques such as ELISA, Western blotting, immunohistochemistry, flow cytometry, and affinity chromatography. The extended spacer arm (XX linker) is particularly beneficial when labeling large proteins or when the biotin binding site might be partially obscured. The compound is widely used in the development of biotinylated probes, biosensors, and targeted drug delivery systems. It is a key tool in proteomics for labeling and capturing protein complexes. Handling requires standard precautions for organic compounds, and the NHS ester form must be protected from moisture to prevent hydrolysis.