Development of arthritis in EP receptor–deficient animals. (a) Severity of arthritis. Mice bearing targeted mutations for each of the individual EP receptors were tested for their ability to develop clinical signs of arthritis. Paws from each mouse were scored on day 10 as described in Methods. EP1+/+, n = 7; EP1–/–, n = 10; EP2+/+, n = 10; EP2–/–, n = 10; EP3+/+, n = 8; EP3–/–, n = 8; EP4+/+, n = 24; EP4–/–, n = 23. *P < 0.05 by Mann-Whitney U test. (b) Incidence of arthritis. *P < 0.05 by χ-square test.