Questiomycin A is a phenoxazine and a chromophore that has been found in Streptomyces and has antibacterial and anticancer activities.1,2,3,4,5 It is active against M. scrofulaceum, M. marinum, and M. intracellulare (MICs = 2.8, 11.3, and 5.6 μg/ml, respectively) but not M. tuberculosis, M. smegmatis, M. kansasii, or M. fortuitum (MICs = >45 μg/ml).3 It is also inactive against E. coli, P. aeruginosa, S. tymphimurium, S. aureus, or L. monocytogenes. It is cytotoxic to a variety of cancer cells, including MCF-7, A549, MIA PaCa-2, and LoVo-1 cells (IC50s = 1.67, 5.48, 7.16, and 20.03 μM, respectively) as well as human umbilical vein endothelial cells (HUVECs) but not human embryonic lung fibroblast cells (HELs; IC50s = 16.06 and >50 μM, respectively).4 Questiomycin A reduces the increased intracellular pH in a variety of cancer cell lines, as well as in HUVECs and HELs. It prevents lung metastasis in a B16 mouse melanoma model of metastasis when administered at a dose of 0.5 mg/kg simultaneously with B16 cells or every three days.5 It is also a chromophore product of the reducing agent 2-aminophenol oxidation (as 2-amino-phenoxazine-3-one) and has been used as a readout in the study of catalytic oxidation of 2-aminophenol by various metal-containing complexes.6,2 It has an absorbance of 435 nm in methanol.2