A polyhistidine-tag (His-tag) is an amino acid motif in proteins that consists of at least six histidines and is commonly positioned at the N- or C-terminus of the protein. His-tag is a popular tag used for purification of proteins by Ni-NTA affinity chromatography. The use of this tag is nowadays extended also to surface immobilization, incorporation of active proteins in nanomaterials and Protein Labeling with biotin and Fluorophores, as with NanoTemper Technologies' RED-tris-NTA labeling kits.