P2X2 receptors were originally cloned from rat pheochromocytoma cells, the tumor counterpart of chromaffin cells [63], and together with P2X1, P2X3, P2X5 and P2X7 receptors were identified by immunohistochemistry in chromaffin cells from the same species [64,65,66]. This evidence concerns the gene P2RX1 and hereditary pheochromocytoma-paraganglioma.