Gene expression in 18 tissues. Numbers of genes detected in each tissue are compared between microarray and EST data (A). Tissues are ranked from the poorly-sampled (left) to the highly-sampled (right) according to the EST data. The numbers of detected genes are plotted against the numbers of sampled ESTs for the 18 tissues (B). The sampling growth curve is fitted by Hill function f(x) = axb/(c+xb) with a = 17622.8, b = 0.8, c = 6259.7. The curve indicates that current transcriptome sampling is far from saturated. Percentage of genes is plotted against the number of tissues where they express to give the expression breadth distribution (C). Expression breadth in microarray data is compared against that in EST data, with color from white to blue indicating the number of incidence from low to high (D). The correlation of expression breadths between the two types of data is not significant (r = 0.42); 71% of the genes are detected in less number of tissues by microarray data than by EST data.