Exploiting the ability of divalent phosphate to coordinate two such groups, MIPs can be prepared featuring cleft-like receptors with tight affinity for the template or target epitopes. In contrast to most other phosphopeptide enrichment techniques, these receptors are charge neutral and therefore less biased towards peptides comprising other charged amino acid residues, such as Glu and Asp. To apply this technique to the enrichment of specific phosphopeptides we focused on Zetachain associated protein kinase 70 kDa (ZAP-70), a well-characterized protein playing a central role as mediator in the T-cell receptor (TCR) signalling cascade. ZAP-70 is inactive cytosolic tyrosine kinase which associates with immunoglobulin tyrosine-based activation motifs (ITAMs) on the transmembrane TCR via two SH2 domain modules. These are connected to the kinase domain via a flexible interdomain (Scheme 1a).