P2RX4 and age-related macular degeneration: The role of P2X7 on MPs in AMD is further indicated by genetic studies which suggest inheritance of a loss of function P2X7 receptor variant combined with P2X purinoceptor 4 (P2X4) variant increases the risk of developing AMD, likely due to a reduction in the ability of peripheral blood monocytes and microglia to phagocytose debris (Gu et al., 2013).