Identification of the corresponding prostanoid receptor subtypes within the synovial tissue of these patients revealed that the EP3 prostanoid receptor subtype was the most abundant in all three groups of patients (RA patients: 10-fold; OA patients: 11-fold; and JT patients: 5-fold) followed by the other prostanoid receptor subtypes (EP1-EP3) which differed not much among each other (RA: ca. 5-fold; OA: ca. 4-fold; and JT: ca. 3-fold). Here, PTGER1 is linked to rheumatoid arthritis.