The last two decades have seen the development of an expanding array of monoclonal antibodies, which are proving to be effective treatments for both cancer and autoimmune disease. A related protein, the human T-cell receptor (TCR), is able to access a new range of targets for which antibodies are unsuitable, and is being developed as a new class of protein therapeutics. Similar to antibodies, TCRs have a specific target and therefore a limited potential for side effects; however, a possible drawback with these receptors is that their natural affinity is low. This problem has recently been overcome using phage display to increase TCR affinity up to a million-fold.