Gene expression in the 25K Populus cDNA array and in the POPULUSDB. The 50 most highly expressed transcripts in the microarray analysis of xylem fibers undergoing cell death are shown together with the corresponding library distribution in POPULUSDB. The transcripts are listed in descending order of expression ratio between the fiber-cell death sample and the early fiber development sample (see Figure 1). The color code at the bottom of the figure indicates the number of ESTs in each library for each transcript. The whole list is given in Additional data file 2.