The effect of different normalization factors on the relative gene expression levels of Sox9 and insulin-like growth factor-1 (IGF-1). The expression level of Sox9 is presented after chondrogenic induction relative to values at passage 0. The values have been normalized to either the geometric mean (Geom mean) of tyrosine 3/tryptophan 5-monooxygenase activation protein (YMHAZ), TATAA-box binding protein (TBP), and beta-glucuronidase (GUSB), to 18S RNA, or to beta-actin (ActB). The effect of hypoxic culture on expression of IGF-1 is presented after the values have been normalized to either the geometric mean, GAPDH, or ActB levels. Error bars denote standard error of the mean, and asterisks indicate p < .05 compared with values normalized to the geometric mean.