For each gene in each microarray hybridization experiment, the ratio of RNA levels from the two samples was transformed into a log2 value and the mean log2 ratio was calculated. The log2 ratios were normalized by print-tip Loess normalization (Dudoit and Yang, 2002). All genes with a false discovery rate of <= 5% (q <= 0.05) (Storey and Tibshirani, 2003) and a mean fold-change ratio of >= 1.5 were selected for further analysis.