To measure tissue bias, we used: (1) the statistic t [61] , which represents the average difference in normalized expression intensity from that of the tissue of maximal expression, and (2) a statistic, here denoted c, defined as c = max t c t , where c t is the squared cosine of the angle between the expression vector and the coordinate axis associated with t (see [17] ). In defining genes as tissue specific for tissue t we required c t .0.25 and c t9 ,0.125 for all t9 ? t. Further details are given in Text S1.