A rare enzyme deficiency which is generally asymptomatic but can cause problems when certain anti-cancer or immunosuppressant drugs are used (e.g. 6-mercaptopurine, 6-thioguanine, azathioprine). The body is unable to metabolize these drugs which leads to a toxic buildup which can ultimately lead to reduced blood production by the bone marrow. See also Thiopurine S methyltranferase deficiency