P2RX7 and bipolar disorder: Although the P2RX7 variants fell just below the significance threshold and did not replicate in the British sample, it is worth noting that the gene has previously been associated with bipolar disorder,39, 47 including a study of a British sample overlapping with the present sample, showing association with another P2RX7 variant.8 The gene belongs to a family of purinoreceptors for ATP, which function as ligand-gated ion-channels, and seem to have a role in the ATP-induced glutamate transmission in the hippocampus.48