- Genetic sequence of drug-metabolizing enzymes, transporters/receptors, transcription factors, drug targets, and patient baseline characteristics often affect an individual s response to drug therapy. Expression of such genes is also influenced by the epigenome and regulation by a variety of other factors: RNA expression, protein expression, disease state, comorbidities, concomitant therapies, etc. Therefore, inter-patient variability in drug pharmacokinetics and outcome is often a function of these factors.