Consistent with previous observations (e.g., [62] ), we found a significant negative correlation of v with expression level in all 11 tissues (Spearman's rank correlation coefficient r ranged from 20.25 to 20.43). In addition, we observed a positive correlation of v with tissue bias, as measured by t (r = 0.24) [63, 64] . (Similar correlations were observed when the log likelihood ratio in the test for positive selection on any branch-which increases with increasing evidence for selection-was used in place of v.) Unlike in previous studies, however, we were able to examine these correlations separately for positively and non-positively selected genes, using the set of PSGs identified by the all-branches LRT. Interestingly, the correlations of v with expression level t are much stronger within the non-PSGs than within the PSGs, indicating that the observed correlations are primarily driven by negative rather than positive selection (see also [65] ). Thus, while genes expressed at low levels and/or in a tissue-specific manner show an increased tendency to have experienced positive selection, the strength of positive selection does not appear to be strongly correlated with their expression patterns (see Discussion).