In summary, 19-substituted BQAs did not react with thiols,demonstratedlower rates of redox cycling and were markedly less toxic to normalhuman cells and to mouse hepatocytes when compared to the parent compounds.In cancer cells, 19-phenyl-substituted analogues were less activecompared to unsubstituted BQAs and similar to the parent BQAs 19-phenylanalogues were more active in the presence of NQO1 suggesting a rolefor intracellular reduction to the hydroquinone in promoting growthinhibition. Here, NQO1 is linked to cancer.