Riboflavin kinase (RFK; EC 2.7.1.26) is an essential enzyme that catalyzes the phosphorylation of riboflavin (vitamin B2) to form flavin mononucleotide (FMN), an obligatory step in vitamin B2 utilization and flavin cofactor synthesis (Karthikeyan et al., 2003 [PubMed 12623014]).[supplied by OMIM, Nov 2009].