Figure 2. Enrichment of bHLH family in early phase of osteoblast differentiation and their expression pattern.

(A) Summary of up-regulated transcription factors (Figure S1) associated with five phases of osteoblast differentiation. Asterisk (*): bHLH transcription factors that are overrepresented only during the early phase of osteoblast differentiation (p<0.01, Figure 2B). Sharp (#): transcription factors known to be associated with osteoblast differentiation and/or bone development. (B) Chi-square (χ²) test for independence of bHLH superfamily genes in the immediate early phase (1 hr) of osteoblast differentiation. Null hypothesis: the occurrence of up-regulated bHLH transcription factors observed during the immediate early phase of osteoblast differentiation among all genes associated with probe set of Affymetrix GeneChip Mouse Genome 430 2.0 array is independent. Results of the contingency table (2×2 cells, degree of freedom = 1) allowed to reject the Null hypothesis. The expression of bHLH transcription factors was significantly up-regulated during the immediate early phase of osteoblast differentiation (p<0.01). (C) Expression pattern of six bHLH family genes in osteoblast and adipocyte differentiation. The chart represents expression changes of six bHLH genes during osteoblast and adipocyte differentiation. The horizontal axis indicates each time point after induction. The vertical axis indicates the logarithmic expression ratio (base 2) of each time point in comparison to the control (0 day).