Catalyzes the phosphorylation of riboflavin (vitamin B2) to form flavin-mononucleotide (FMN), hence rate-limiting enzyme in the synthesis of FAD. Essential for TNF-induced reactive oxygen species (ROS) production. Through its interaction with both TNFRSF1A and CYBA, physically and functionally couples TNFRSF1A to NADPH oxidase. TNF-activation of RFK may enhance the incorporation of FAD in NADPH oxidase, a critical step for the assembly and activation of NADPH oxidase. ATP + riboflavin = ADP + FMN + H(+) Zinc or magnesium. Cofactor biosynthesis; FMN biosynthesis; FMN from riboflavin (ATP route): step 1/1. Monomer. Directly interacts with TNFRSF1A death domain. TNFRSF1A-binding may be supported by TRADD. In the absence of TNFRSF1A, interacts with TRADD. Independently of TNFRSF1A, interacts with the NADPH oxidase subunit CYBA. Detected in brain, placenta and urinary bladder. Extended N-terminus. Extended N-terminus.