To calculate a P-value for each gene6tissue, the DABG Pvalues of all associated probesets (pooling the three replicates per probeset6tissue) were combined using Fisher's method [89] . A gene was considered to be significantly expressed above background if it had (nominal) P,0.001. Similarly, an estimated expression intensity for each gene6tissue was calculated by first taking the median over the three replicates of each RMA-based probeset summary, then taking the median of these values over all probesets associated with the gene. The analysis of expression intensities was restricted to genes significantly expressed above background so that genes expressed at or near the background level did not drive the results.