Effect of irisin on the expressions of osteogenic genes and markers in MC3T3-E1 cells after exposing MC3T3-E1 cells to 100 ng/ml irisin for 21 days. *P<0.05; **P<0.01 vs. control. ALP, alkaline phosphatase; RUNX2, runt-related transcription factor-2; COL-1, collagen I; OSX, osterix; OPN, osteopontin; OCN, osteocalcin; OPG, osteoprotegerin; ERα, estrogen receptor α.