Gene Ontology organizes gene function into hierarchical categories based on biological process, molecular function and cellular component (22) (23) (24) . Fisher's exact test was applied for over representation of selected genes in GO biological categories. In order to assess the significance of a particular category by random chance, false discovery rate (FDR) was estimated for all of categories. After 5,000 re-samplings, FDR was defined as FDR=1-Nk/T, where Nk refers to the subtracted number which was from Fisher's test p-values minus ϰ 2 test p-values in random samples. We specified the threshold of significant GO as p-value<0.05, FDR<0.05 and enrichment parameters. Enrichment represents the degree of gene expression significance. The equation of enrichment is as following Re=(nf/n)/(Nf/N) where nf is the number of significant genes within the particular category, n is the total number of genes within the same category, Nf is the number of significant genes in the entire microarray, N is the number of all genes tested.