P2RX4 and myalgic encephalomeyelitis/chronic fatigue syndrome: Postexercise severity of clinical pain and fatigue ratings in CFS-only and CFS+FMS patients were correlated with increases in gene expression, especially with P2X4, α-2A and β-2 receptors (r  =  +0.43, +0.48, and +0.44 for pain, and r  =  +0.51, +0.60, and +0.47 for fatigue, resp.; P  <  0.01) [89].