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The promise of pharmacogenetics is to elucidate the inherited basis of differences between individual responses to drugs, in order to identify the right drug and dose for each patient. Genetic polymorphisms are implicated in the interindividual variability of the pharmacokinetic or pharmacodynamic characteristics of immunosuppressive drugs. The first pharmacogenetic trait identified was monogenic, and concerned the prototypic example of thiopurine methyltransferase (TPMT) implicated in azathioprine metabolism. Individuals with low TPMT activity, inherited in an autosomal codominant fashion, are at risk of drug-induced myelosuppression. TPMT activity determination and DNA-based tests are now used in clinical practice. It has been also demonstrated that there is a link between the polymorphisms of the cytochrome P450 3A5, 3A4 and the multidrug resistance-1 (MDR1) genes, and the daily dose necessary to achieve adequate blood tacrolimus levels. Analysis of MDR1 haplotypes or using the association of the different genes might further improve predictions. Since genotyping methods improve rapidly, it will soon be easy to test for thousands of single nucleotide polymorphisms in one assay. Present challenges are to determine the genes of interest and to validate such determination prospectively in clinical practice.