Abstract: A target-binding polypeptide having (a) a stable core polypeptide region (SCR); and (b) at least one target-binding region (TBR), in which the target-binding region(s) are covalently attached to the SCR and which have optionally been subjected to a maturation step in order to modify the specificity, the affinity or the avidity of binding to the target. The polypeptides may self associate to form stable dimers, aggregates or arrays. The polypeptides of the invention have utility in the diagnostic, therapeutic, predictive or preventative fields of the pharmaceutical and health care industries, as well as more general application in the detection and assay of chemical entities.