Biotin-azide is a form of biotin with a terminal azide group. It is used to add biotin to other molecules that bear either an alkyne group, through click chemistry, or a phosphine group, using Staudinger ligation. Biotin-azide has been commonly used to biotin-tag alkynylated lipids, particularly those associated with proteins through post-translational modification. It can also be used to biotin-tag proteins, lipids, carbohydrates, and nucleic acids that have been modified with alkyne or phosphine groups.