if the two entities are proteins, often linker (or "spacer") peptides are also added, which make it more likely that the proteins fold independently and behave as expected. especially in the case where the linkers enable protein purification, linkers in protein or peptide fusions are sometimes engineered with cleavage sites for proteases or chemical agents that enable the liberation of the two separate proteins. this technique is often used for identification and purification of proteins, by fusing a gst protein, flag peptide, or a hexa-his peptide (6xhis-tag), which can be isolated using affinity chromatography with nickel or cobalt resins. di- or multimeric chimeric proteins can be manufactured through genetic engineering by fusion to the original proteins of peptide domains that induce artificial protein di- or multimerization (e.g., streptavidin or leucine zippers). fusion proteins can also be manufactured with toxins or antibodies attached to them in order to study disease development. hydrogenase promoter, p, was studied constructing a p promoter-"gfp" fusion by using green fluorescent protein ("gfp)" reporter gene.