Defects in ADA^−/− OB activity. (A) In vitro ADA^−/− OBs grow significantly less than wild-type cells. *P = .005-.05. (B) Viability as assessed by the Alamar Blue staining is significantly lower in cultured ADA^−/− OBs. (C) The percentage of apoptotic cells as assessed by annexin V⁺/7-amino-actinomycin D⁻ staining is significantly higher in ADA^−/− OBs but normalizes in cells transduced with a lentiviral vector encoding for ADA (ADA^−/− TR). **P = .001-.005. (D) TaqMan gene expression analyses for osteoblastic differentiation markers Runx2, type 1 collagen, alkaline phosphatase, and osteocalcin as well as RANKL and OPG; fold expression versus wild-type (represented by dashed line), normalized for HPRT endogenous control; average of 3 independent experiments ± SD. (E) Growth of murine ADA^+/+ OB after 12 days in vitro is reduced when cultured with EHNA (100 μM) and abolished when cultured with EHNA and adenosine (1 mM). ADA^−/− OBs regain growth after lentiviral vector transduction. Growth as percentage of initially plated OBs ± SD. (F) Growth of human OB after 13 days of in vitro is reduced when cultured with EHNA (100 μM). **P < .01.