Biotin, a 244 Dalton vitamin found in tissue and blood, binds with high affinity to avidin and streptavidin protein. Since biotin is a relatively small molecule, it can be conjugated to many proteins or nucleic acids without significantly altering their biological activity. Biotinylation reagents are available for targeting a variety of specific functional groups, including primary amines, sulfhydryls, carboxyls and carbohydrates that lead to nucleotides or amino acid biotinilation.