This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
Thiopurine S-methyltransferase deficiency is an autosomal recessive disorder that affects the body’s ability to metabolize thiopurine drugs. Thiopurine S-methyltransferase (TPMT) is an enzyme that the body uses to break down thiopurine drugs. Thiopurine S-methyltransferase deficiency patients have a mutation in either one or both copies of the TPMT gene that causes reduced enzyme activity and difficulties breaking down thiopurine drugs. Affected individuals are at a high risk of experiencing bone marrow damage if they take thiopurine drugs. Many patients recover from the bone marrow damage once thiopurine drug use is stopped. However when the bone marrow is damaged, infections can cause serious health problems and may be life-threatening. To reduce the risk of bone marrow damage, thiopurine therapy should be lowered based on how many mutations a patient has in the TPMT gene. 
For more information, visit GARD.