Questiomycin A is a phenoxazine produced by several Streptomyces species and some fungi and bacteria. Questiomycin A is weakly active against bacteria, fungi, plants and tumor cell lines, and inhibits aromatase and sulfatases. Questiomycin, like other phenoxazines, stimulates cell growth and turnover in vitro, an activity possibly related to their ability to form stable free radicals. More recently, questiomycin A has been shown to inhibit pulmonary metastasis caused by mouse melanoma cells. Questiomycin A and related phenoxazines are important dereplication standards in discovery research to eliminate leads due to high amounts of weakly potent actives.