We have cloned a novel K+-selective, inward rectifier channel that is widely expressed in brain but is especially abundant in the Purkinje cell layer of the cerebellum and pyramidal cells of the hippocampus. It is also present in a wide array of tissues, including kidney and intestine. The channel is only 38% identical to its closest relative, Kir1.3 (Kir1-ATP-regulated inward rectifier K+ [ROMK] family) and displays none of the functional properties unique to the ROMK class. Kir7.1 has several unique features, including a very low estimated single channel conductance (approximately 50 fS), low sensitivity to block by external Ba2+ and Cs+, and no dependence of its inward rectification properties on the internal blocking particle Mg2+. The unusual pore properties of Kir7.1 seem to be explained by amino acids in the pore sequence that differ from corresponding conserved residues in all other Kir channel proteins. Replacement of one of these amino acids (Met-125) with the Arg absolutely conserved in all other Kir channels dramatically increases its single channel conductance and Ba2+ sensitivity. This channel would provide a steady background K+ current to help set the membrane potential in cells in which it is expressed. We propose that the novel channel be assigned to a new Kir subfamily, Kir7.1.