Upper panel: Regions constituting the pore helix and the selectivity filter are shown in the crystal structure of TWIK1. Extracellular and intracellular domains of TWIK1 are not included. Pore helices (PH) are in blue and selectivity filters (SF) are in green. The side chains of residues affecting ion selectivity (L108, F109, T118, H122, L228, T250) are in red. Lower panel: sequence alignment of the pore domains of human and Caenorhabditis elegans K2P channels. Unusual residues are in dark bold.