Department of Physiology, University of Pennsylvania, Philadelphia, PA 19104Recently, CALHM1, a gene of previously unknown function, was identified as a risk factor for late-onset Alzheimer‘s disease (AD). It was suggested that CALHM1 might be an ion channel or ion channel regulator. We used two-electrode voltage clamp (TEV) to measure CALHM1-induced ionic conductances in the plasma membrane of Xenopus oocytes. To investigate which ions permeate the CALHM1-induced conductance, we employed instantaneous current/voltage protocols to measure reversal potentials (Erev) in various solutions. In the absence of extracellular divalent cations, CALHM1 is weakly cation selective, with PNa : PK : PCl = 1 : 1.11 : 0.52. In CALHM1 injected oocytes, the relative permeability among monovalent cations (with respect to Na+) is PRb = PCs = PK > PNa > PLi > PNMDG, indicating that CALHM1-induced permeability is relatively nonselective for monovalent cations. CALHM1 induced currents are divalent cation selective, with PNa : PCa : PBa : PMg = 1 : 13.8 : 8.6 : 3.1. This selectivity sequence represents a Sherry III/IV sequence, consistent with a weak-field strength site in the permeation pore. The presence of extracellular divalent cations strongly alters the gating properties of CALHM1 induced currents. However, the presence of extracellular divalent cations only slightly alters the relative permeability of K+ and Cl- (PNa : PK : PCl = 1 : 1.46 : 0.88). Using large organic cations and a volume exclusion model, we estimated the radius of the CALHM1 associated ion channel pore in the absence of divalent cations to be 6.9 to 8.7 A. (Supported by NIH GM/DK56328 and a pilot project from the University Pennsylvania AD Core Center.)