The kainate receptor subfamily consists of five members that have been further subdivided into two classes based upon structural homology and functional characteristics. GluR5, GluR6, and GluR7 receptor subunits share a high degree of homology and are able to form functional channels when expressed in heterologous systems. The KA-1 and KA-2 receptors are unable to form functional channels on their own, but when coexpressed with GluR5-7 receptor subunits, they form channels with high affinity for kainate. Like AMPA receptors, the functional unit of endogenous kainate receptors is believed to be a tetramer, which can be either homomeric or heteromeric. Kainate receptors GluR5 and GluR6 (but not GluR7, KA-1, or KA-2) can undergo RNA editing; as in the AMPA receptor GluR2, a glutamine (Q) residue in the channel pore is edited to encode arginine (R) in the mature protein. Substitution of Q with R modulates the properties of the channel, producing channels with reduced single channel conductance and lower permeability to Ca²⁺.