P2X receptors are ligand-gated ion channels that induce the flux of cations across the membrane when activated by ATP, resulting in membrane depolarization and Ca 2+ entry [1][2][3][4]. In mammals, P2X receptors have been shown to play a critical role in regulating physiological processes in a wide range of cell types such as neurons, glial cells, muscle, endothelial and epithelial cells, osteoclasts, and hematopoietic cells [1,3]. The cloning of the first P2X receptor P2X 1 in 1994 revealed an unusual structure of P2X receptors containing two transmembrane segments [5,6], which is distinct from other ionotropic receptors such as glutamate receptors and acetylcholine receptors [3]. P2X receptors appear to be present in all vertebrate and many invertebrate genomes but are absent in some invertebrates such as Caenorhabditis elegans and Drosophila melanogaster [4,7,8]. It should be noted that C. elegans and D. melanogaster are also known to lack two other Ca 2+ -permeable channelsthe voltage-gated CatSper Ca 2+ channels at the plasma membrane [9] and the two-pore channels underlying NAADP-sensitive Ca 2+ release from acidic Ca 2+ stores [10].ATP is widely utilized as an energy source in many organisms, and so, ATP could serve as an evolutionarily conserved means to mediate cell-cell communications or intracellular signaling through P2X receptors. The emergence of purinergic signaling by P2X receptors is believed to have occurred in early evolution of eukaryotes [4,8], which is supported by the identification and characterization of P2X receptor homologs in non-metazoan organisms such as the dictyostelid social amoeba Dictyostelium discoideum [11], the green alga Ostreococcus tauri [12], and the choanoflagellate Monosiga brevicollis, one of the closet unicellular relatives of animals [12,13]. Interestingly, even though a P2X receptor homolog is characterized in D. discoideum [11], P2X receptor homologs have not been detected in known genomes of fungal species [4,8]. It is generally believed that P2X receptors had been lost in fungi [4,8].We have recently demonstrated in the choanoflagellate M. brevicollis the presence of extensive Ca 2+ signaling and amplification pathways, most of which are previously believed to be animal specific [13]. To further explore the origin of the Ca 2+ signaling machinery, we have more recently reported the examination of several genomes at the Origins of Multicellularity Database [14,15], the Broad Institute and the NCBI genomic databases including the genomes of three basal fungi Allomyces macrogynus, Spizellomyces punctatus, and Batrachochytrium dendrobatidis [16]. Surprisingly, our analysis revealed the presence of P2X receptor homologs in the three basal fungi (AmaP2X, SpuP2X, and BdeP2X) ( Fig. 1; Electronic supplementary Electronic supplementary material The online version of this article