Our analysis identify a striking difference in the level of miRNA regulation between PK and PD genes, with the former having less than half predicted conserved miRNA binding sites compared with the latter. Importantly, this finding is reflected in a highly significant difference in the shift in expression levels of PD versus PK genes after depletion of miRNAs. CONCLUSIONS: Our study emphasizes an intrinsic difference between PK and PD genes and helps clarify the role of miRNAs in pharmacogenomics.