The P2X4 receptor is a member of a family of ATP-gated cation channels, which are composed of three subunits, each having two transmembrane domains linked by a large extracellular loop and intracellularly located N- and C-termini. Recent crystallization of zebrafish P2X4 not only provided the possibility for re-interpretation of data generated by site-directed mutagenesis but also to progress in further work on structural-functional characterization.