Previous studies in fly (Dopman and Hartl, 2007) and mouse (Henrichsen et al., 2009) have shown that CNV genes tend to be more specific in their tissue expression patterns. We investigated this phenomenon in cattle using gene expression data from 22 different tissues (Table 2). Expressed CNV genes were expressed in fewer tissues (median = 2) than expressed neutral genes (median = 10) (one-tailed Wilcoxon rank-sum test, P < 0.00001). This is consistent with results from a similar study in fly (Dopman and Hartl, 2007), suggesting that CNVs occur more often in genes with tissue-specific expression than widely expressed genes that may have housekeeping functions.