Arabidopsis genes encoding plant-specific proteins exhibit preferential expression in organs. (a) Heat map showing the 600 plant-specific genes that exhibited differential expression in at least one microarray experiment comparing RNA samples from different plant organs. Microarray experiments were obtained from the Stanford Microarray Database. The mean was calculated for the replicates. Organ preferential expression was defined as a twofold or higher ratio in the comparison. Gene expression is expressed as the log2(ratio). The bar at the top right indicates the magnitude of change. Green indicates induction and red indicates depression of gene expression. Ref, reference sample; see Materials and methods for details. (b) For all organ comparisons the number of differentially expressed genes in the plant-specific category was statistically higher than the number of differentially expressed genes that are not plant specific. Calculation of the statistical significance was done using the chi-square test for contingency tables.