Morphologic findings of cultured cells and relative expression levels of mRNA of transcriptional factors. Cultured cells in OLF (a) and control (b) showed spindle shape (scale bar = 100 μm). RT-PCR analysis of the ossified and nonossified cultured ligament cells showed that the mRNA expression levels of Sox9, Runx2, Msx2, and AP-1 in OLF were significantly different from those of the control (c). Data are expressed as mean ± SEM values of six samples each. Differences between the two groups were examined with the Student t test. *P < 0.05, compared with the control. mRNA, messenger RNA; OLF, ossification of the ligamentum flavum; RT-PCR, reverse transcription-polymerase chain reaction.