PDZ domains are conserved protein domains of about 90 amino acids involved in protein–protein recognition, protein targeting and assembly of multi-protein complexes. The name PDZ derives from the first three proteins in which these domains were identified: PSD-95 (a 95 kDa protein involved in signaling at the post-synaptic density), DLG (the Drosophila melanogaster Discs Large protein) and ZO-1 (the zonula occludens 1 protein involved in maintenance of epithelial polarity). The multidomain nature of PDZ-containing proteins enables them to interact with multiple binding partners and hence organize larger signaling protein complexes.