The P2X1 receptor belongs to a family of oligomeric ATP-gated ion channels with intracellular N and C termini and two transmembrane segments separating a large extracellular domain. Here, we describe a naturally occurring dominant negative P2X1 mutant. This mutant lacks one leucine within a stretch of four leucine residues in its second transmembrane domain (TM2) (amino acids 351-354). Confocal microscopy revealed proper plasma membrane localization of the mutant in stably transfected HEK293 cells. Nevertheless, voltage-clamped HEK293 cells expressing mutated P2X1 channels failed to develop an ATP or ADP-induced current. Furthermore, when co-expressed with the wild type receptor in Xenopus oocytes, the mutated protein exhibited a dose-dependent dominant negative effect on the normal ATP or ADP-induced P2X1 channel activity. These data indicate that deletion of a single apolar amino acid residue at the inner border of the P2X1 TM2 generates a nonfunctional channel. The inactive and dominant negative form of the P2X1 receptor may constitute a new tool for the study of the physiological role of this channel in native cells.