Citreoindole is an unusual diketopiperazine metabolite first isolated from a hybrid cell fusion of two strains of Pencillium citreoviride by researchers at Keio University, Japan in 1991. In 2016, Capon and co-workers revised the structure of citreoindole from the 5-benzyl-4-imidazolidyl ring to a 6-phenyl-4(1)-pyrimidone ring. The same year, its stereochemistry was resolved by Gloer and co-workers by extrapolation from the crystal structure of a close analogue, haenamindole. Citreoindole has weak activity against mammalian tumor cell lines but its pharmacology has not been further investigated.