Polymorphisms of genes encoding the different enzymes involved in the metabolism of AZA and in particular, TPMT may imply an increased risk of adverse effects during treatment [27], especially agranulocytosis, which is a rare but potentially lethal side effect of the drugs that justifies the screening for TPMT before starting AZA treatment [28,29]. The gene discussed is TPMT; the disease is Absence of circulating granulocytes.