Biotin-PEG6-Mal is a maleimide biotinylation linker that can react with sulfhydryl groups (at pH 6.5-7.5) specifically to form a thioether linkage. The hydrophilic PEG spacer increases aqueous solubility of the molecules conjugated to the biotin compound. It also helps to minimize steric hindrance involved with the binding to avidin molecules.