Graphic representation of the mammalian post-synaptic density, with the origination period of the genes indicated by the same colours as in Fig. 2. Domain structures are represented only for scaffold proteins. For most PDZ-binding proteins, the PDZ ligand has been labelled in grey, because the extent of sequence variability observed among PDZ ligands across taxa and proteins is so high that it is generally impossible to conclude either for or against the presence of a PDZ ligand (except for CRIPT and δ-cat; see Additional file 4). A darker red colour has been used for the Shank SAM domain, as it might not have been present in the Shank protein of the common choanoflagellate-metazoan ancestor (see text). Note that the general configuration of the post-synaptic scaffold has been completely re-drawn with respect to  (their Fig. 1), to account for data available in the recent literature (e.g. [17, 18]).