PTGER4 and ankylosing spondylitis: An important role for EP4 receptor could be suggested since polymorphisms in PTGER4 loci are associated with increased PTGER4 gene expression in synovial biopsy samples from patients with spondyloarthritis, and PTGER4 is a susceptibility gene for ankylosing spondylitis and RA (Evans et al., 2011; Rodriguez-Rodriguez et al., 2015).