Data analysis and data normalization were performed using the method described by Quackenbush (8) . For background correction, the mean intensities of areas surrounding spots were subtracted from spot intensities (local area background). Data sets were normalized by dividing the mean intensity value of every spot (in duplicate) by sum of all spot intensities within a sample to eliminate experimental or data acquisition variations. Normalized data were used to calculate the gene expression level ratios of different culture stages. A two-fold expression cut-off was applied. For hierarchical gene cluster analysis, expression ratios were calculated for all genes as described by Eisen et al. (9) .