Defining whether a gene is expressed, and how many genes are detected as expressed per sample. (a) The curves show the cumulative distribution for negative-control probes (cyan line) and for probes on the array (blue line), over all arrays, to illustrate how genes were defined as expressed. The dotted black line indicates the 99thpercentile for the negative control spots. (b) The number of genes expressed in any given number of tissues (between 1 tissue and 55 tissues; for example, there are 4,475 genes detected in only one sample, 171 genes expressed in exactly 27 samples, 1,790 genes detected in all 55 samples, and so on). The genes expressed in each of the 55 tissues were determined as in (a). (c) Number of genes defined as expressed in each of the 55 tissues, using criteria in (a).