Determination of number of clusters. The initial cluster number for each group is determined based on the change trend between average correlation coefficient of expression values of genes in the same clusters and average size of clusters (i.e. number of genes in the cluster). The rectangle fields show the drastic change on correlation coefficients and cluster sizes. Here, group 1 represents the DEGs based on the comparison at 12 hours; group 2 represents the DEGs based on the comparison at 24 hours; group 3 represents the DEGs based on the comparison at 48 hours; overlap represents the genes differentially expressed based on the comparisons at all three time points.