EP2, EP4, cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) were significantly increased in endometriosis lesions compared to the peritoneum of naive mice (NP) or mice with endometriosis (EP; p < 0.01; Supplementary Fig. 2a–d); EP2, EP4, COX-1 and COX-2 proteins were immunolocalised to glandular and stromal cells in lesions and mesothelial cells in the peritoneum (Supplementary Fig. 2). Here, PTGER4 is linked to endometriosis.