The expression levels of putative reference genes normalized to the geometric mean of YMHAZ, TBP, and GUSB. The values for each gene are relative to the expression level in cells at passage 0 (white bars). The yellow bars indicate subsequent passages, where the increasing intensity corresponds to the duration of culture. The blue bars denote values from chondrogenic cultures and the red indicate those from osteogenic cultures. The two light and dark grey bars represent values from cells cultured for one and two weeks in hypoxia, respectively. Abbreviations: ActB, beta-actin; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; YMHAZ, tyrosine 3/tryptophan 5-monooxygenase activation protein; TBP, TATAA-box binding protein; GUSB, beta-glucuronidase; PPIA, cyclophilin A.