Polymorphisms in the human genome contribute to wide variations in how individuals respond to medications, either by changing the pharmacokinetics of drugs or by altering the cellular response to therapeutic agents. The goal of the emerging discipline of pharmacogenomics is to personalize therapy based on an individual's genotype. Due to the relatively large frequency of single-nucleotide polymorphisms (SNP) in the human genome, synonymous SNPs are often disregarded in many pharmacogenomic studies based on the assumption that these are silent. We have shown recently that synonymous SNPs in ABCB1 (P-glycoprotein), which is implicated both in determining drug pharmacokinetics and multidrug resistance in human cancer cells, can affect protein conformation and function. We discuss the importance of polymorphisms in drug metabolizing enzymes and transporters in anticancer therapy and suggest that synonymous polymorphisms may play a more significant role than is currently assumed. [Cancer Res 2007;67(20):9609-12] Should Synonymous Single-Nucleotide Polymorphisms Be Ignored?Clinical observations beginning in the 1950s suggested that individuals exhibit differences in their responses to drugs and that these variations could be inherited (1). Numerous studies over the next few decades clearly established that genetic factors influence the heterogeneity of individual responses to medications with respect to both toxicity and efficacy (1). Although it was recognized that medical practice based on population responses did not reflect the best treatment for an individual (2), the difficulties associated with identifying the genetic determinants of drug sensitivity and toxicity in individual patients were almost insurmountable (1). The dramatic expansion in the development of functional genomics, bioinformatics and high-throughput screening that followed the completion of the human genome project has, however, led many authorities to predict that the dream of personalized medicine may soon be a reality. Analysis of the sequence of the human genome has shown that the extent of genetic variation in the human population is far greater than had been estimated (3), and the most common sequence variation is the single-nucleotide polymorphism (SNP). SNPs (which by definition have a minor allele frequency of >1%) are major determinants of variations in disease susceptibility, response to medication, and toxicity (4). As SNPs occur at a frequency of f1 per 100 to 1000 bp, 3 several strategies have been advocated to systematically or rationally reduce the number of SNPs that need to be studied. Risch, for example, classified SNPs into five types that constitute a hierarchy of importance (see Table 2 in ref. 4). One casualty of this approach has been the synonymous SNPs (which do not alter amino acid residues) based on the assumption that these are ''silent'' and do not affect protein expression or function.Genetic studies have indicated for several years that synonymous mutations can have significant ''fitness consequences'...