Since carboxylesterases have an established and prominent role as drug metabolizing enzymes [21, 26-28], we examined the expression levels of two of its family members; carboxylesterase 1 (CES1) and carboxylesterase 2 (CES2), in bone marrow and/or peripheral blood cells of AML patients at diagnosis. The gene discussed is CES1; the disease is acute myeloid leukemia.