Reduced RANKL/OPG ratio and PINP levels in ADA-deficient mice. (A) Murine serum RANKL (ng/mL) in ADA^+/+ (n = 22) and ADA^−/− (n = 18), Rag2γc^+/+ (n = 10), and Rag2γc^−/− (n = 14) mice. (B) Murine OPG (ng/mL). (C) Murine RANKL/OPG ratio (AU). ***P < .001. **P = .001-.005. *P = .005-.05. N.S. indicates not significant. (D) In vivo bone formation rate as assessed by PINP levels are significantly lower in ADA^−/− mice compared with wild-type controls. Serum PINP levels (ng/mL), ADA^+/+ (n = 9), ADA^−/− (n = 9), Rag2γc^+/+ (n = 15), and Rag2γc^−/− (n = 13).