When Monascus MX was grown under blue light instead of in the dark, citrinin production increased from 478 mg l(-1) to 698 mg l(-1). To explain this, the expression of the pksCT gene, which encodes citrinin polyketide synthase, and of 5 ORFs around it, were monitored. Blue light enhanced citrinin production by upregulating the expression of orf1, orf3, and orf4, indicating that pksCT was not the key gene responsible for the quantity of citrinin production in blue light.