ELISA for OPG levels. Forty-eight hours after transduction of hMSCs with AAV-OPG^wt or AAV-OPG^Y49R or AAV-OPG^F107A culture, supernatants were analyzed by ELISA for OPG levels using a commercial kit (A). OPG levels were also quantitated in mouse serum following therapy. Blood samples were obtained from untreated and hMSC-OPG–treated mice both prior to initiating hMSC therapy and 14 days after the treatment and similarly analyzed by ELISA for OPG levels (B). Data presented are mean ± standard error from triplicate experiments (*P < .05; **P < .01).