The P2X(7) receptor is an ATP-gated cation channel expressed in immune cells and plays a role in proinflammatory cytokine release from monocytes and macrophages. This study investigated the coinheritance of 12 functionally relevant single nucleotide polymorphisms (SNPs) in the human P2X(7) gene (P2RX7), and the functional effect of each singly and in combination was assessed by measurements of ATP-induced currents and ethidium(+) uptake. Genotyping of 3430 Caucasian subjects identified 4 common haplotypes in addition to the common (wild-type) P2X(7)-1. Two haplotypes (denoted P2X(7)-2 and P2X(7)-4) contained various combinations of gain-of-function SNPs. P2X(7)-4 was identified uniquely by the Gln-460 to Arg polymorphism (rs2230912). When expressed in HEK-293 cells, recombinant P2X(7)-2, and P2X(7)-4 haplotypes displayed a 3-fold and 5-fold increase, respectively, in receptor function compared to the wild-type P2X(7)-1. Both P2X(7) haplotypes contained the Ala-348>Thr polymorphism (rs1718119), and this mutation was critical for the gain-of-function effect. Peripheral blood monocytes and erythrocytes from subjects homozygous for gain-of-function P2X(7) haplotypes exhibited increased ATP-induced ethidium(+) uptake and (86)Rb(+) efflux, respectively, and this correlated with increased IL-1beta secretion from LPS-primed monocytes. Inheritance of these P2X(7) haplotypes predisposing to increased proinflammatory cytokine secretion may be important in genetic association studies of inflammatory, infectious, and psychiatric disorders.