P2RX4 and age-related macular degeneration: In a study by Gu et al. (2013), they researched P2RX7 and P2RX4 genes in 744 AMD patients and 557 Caucasian controls and reached a conclusion that a rare functional haplotype of the P2RX4 leads to loss of innate phagocytosis and confers increased risk of AMD.