Azathioprine (AZA), 6-mercaptopurine (6-MP), and thioguanine (TG) are thiopurine drugs. These agents are indicated for the treatment of various diseases including hematologic malignancies, inflammatory bowel disease (IBD), rheumatoid arthritis, and as immunosuppressants in solid organ transplants. Thiopurine drugs are metabolized, in part, by thiopurine methyltransferase (TPMT). TPMT displays genetic polymorphism resulting in null or decreased enzyme activity. At least 20 polymorphisms have been identified, of which, TPMT *2, *3A, *3B, *3C, and *4 are the most commonly studied. These polymorphisms have been associated with increased myelosuppression risk. TPMT genotyping may be useful to predict this risk.

Hepatitis C virus (HCV) is a bloodborne infection that is one of the leading causes of liver disease. If left untreated, HCV can lead to cirrhosis, hepatocellular carcinoma, and death. The current standard of care for HCV is a combination of pegylated-interferon (peg-IFN) and ribavirin (RBV) in which the goal of treatment is to decrease complications and death due to HCV. HCV displays genetic polymorphism, where patients with HCV genotype 1 may have higher viral replication rates and are less likely to respond to treatment. These patients require a longer duration of treatment and a higher RBV dose. The interleukin (IL) 28B genotype test is associated with a sustained virologic response (SVR), defined as an undetectable HCV ribonucleic acid (RNA) upon completion of treatment and 24 weeks thereafter.