NQO1 and neoplasm: NQO1 is a flavoprotein that catalyzes 2-electron reduction of quinones and N-oxides.1 Several human diaphorases are known,1,22,23 but NQO1 appears to be most important for activating bioreductive agents.1,22,24 NQO1 is a homodimer that uses NAD(P)H as an electron donor.1 The enzyme is mainly cytosolic, but a significant proportion of the enzyme is present in the nucleus of cancer cells.25 It is ubiquitous in eukaryotes but levels vary in different tissues,1,24,26 with low levels in hematopoetic cells.26 Activity is usually higher in tumor than normal cells.24,26,26