Validation of microarray data. Comparison of the relative expression levels of genes selected from different clusters, as determined by microarray (closed symbols) and qPCR (open symbols). qPCR data were normalized to HPRT mRNA concentration and plotted relative to the level at time zero. Data are presented as means ± SD of duplicate experiments. Note that the qPCR assays generally yielded higher mRNA values than the microarray analysis. See also the correlation plot in additional file 6.