 For natural proteins , a series of purification steps may be necessary to obtain protein sufficiently pure for laboratory applications . To <unk> this process , genetic engineering is often used to add chemical features to proteins that make them easier to <unk> without affecting their structure or activity . Here , a " tag " consisting of a specific amino acid sequence , often a series of <unk> residues ( a " His @-@ tag " ) , is attached to one terminus of the protein . As a result , when the lysate is passed over a chromatography column containing nickel , the <unk> residues <unk> the nickel and attach to the column while the <unk> components of the lysate pass unimpeded . A number of different <unk> have been developed to help researchers <unk> specific proteins from complex mixtures . 
