Pannexins are vertebrate proteins with limited sequence homology to the invertebrate gap junction proteins, the innexins. However, in contrast to innexins and the vertebrate connexins, pannexins do not form gap junction channels. Instead they appear to solely function as unpaired membrane channels allowing the flux of molecules, including ATP, across the plasma membrane. We provided additional evidence for their ATP release function by demonstrating that the connexin mimetic peptides, which were thought to inhibit ATP release through connexin channels, do not inhibit their host connexin channels but instead inhibit pannexin1 channels by a mechanism of steric block. Therefore, the inhibitory effects of mimetic peptides on ATP release may represent supporting evidence for a role of pannexin1 in ATP release. We also analyzed the pore structure of pannexin1 channels with the Substituted Cysteine Accessibility Method. The thiol reagents MBB and MTSET reacted with several positions in the external portion of the first transmembrane segment and the first extracellular loop. In addition, MTSET reactivity was found in the internal portion of TM3. These data suggest that portions of TM1, E1 and TM3 line the pore of pannexin1 channels. Thus, the pore structure of pannexin1 is similar to that of connexin channels.