Protein A thiol (sulfhydryl) is a recombinant version of the Staphylococcal protein that has been modified using iminothiolane. These thiols can react with maleimide containing compounds to form a covalent bond. Keep frozen for long-term storage. Avoid repeated freeze/thaw cycles. Larger quantities are available at substantial savings. For additional details, please visit www.proteinmods.com. Product is shipped FedEx overnight on ice packs within continental US. The first item is $30, additional items are shipped at no additional shipping costs.