P2RY4 and dry eye syndrome: P1 receptor can be divided into four subtypes of adenosine A1, A2A, A2B, and A3, while P2X receptors include subtypes of P2X1-7, and eight subtypes of P2Y receptors: P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11-14, of which the A1, A2A, A3, P2X4, P2X7, P2Y1, P2Y2 and P2Y4 receptors play important regulatory roles in the inflammatory response in dry eye (6).