Thiol (SH) group can be linked to the 5’ or 3’ terminus of an oligonucleotide. This modification is used for oligo activation prior to linking to various types of molecules or support. The thiol groups can be used to attach a variety of products to the oligonucleotide. Common thiol-reactive groups include maleimides, halogens, iodoacetamides and pyridyldisulfides. This DNA modification is commonly used for the attachment of enzymes such as horseradish peroxidase or alkaline phosphatase.