AMPA receptors are permeable to cations Na+, K+ and Ca2+. The Ca2+ permeability is dependent on the presence of AMPA2: whenever this subunit is present, the channel will be impermeable to Ca2+. The Ca2+ permeability of the AMPA2 subunit is determined by the presence of the amino acid arginine (R) at a critical site in the pore loop instead of glutamine (Q) present in the same site in the other AMPA subunits. A post-transcriptional process known as RNA editing determines the presence of this R. Since most AMPA2 subunits in the adult brain have undergone RNA editing and most AMPA receptors contain the AMPA2 subunit, most native AMPA receptors will be impermeable to Ca2+.