Principal component analysis (PCA). A) PCA was performed for gene subsets with > 1.5-fold change in expression over the 28-day culture period (See Methods). The results are shown for the three major gene profiles generated by PCA. The red line represents the mean value of time-course changes (days 7, 14, 21 and 28). B) Expression values were normalized to the mean values of Day 7 samples. The resultant log2 ratios at each day were averaged and displayed in the heatmap. The color bar indicates log2 ratio of change in gene expression.