Otto Warburg and his collaborators were the first to isolate a “yellow ferment†from yeast cells (Warburg and Christian 1933). The yeast genome contains 68 genes encoding for a flavin-dependent protein and thus 1.1% of all yeast protein (5885 protein-encoding genes, Goffeau et al. 1996) have a requirement for either FMN or FAD (Macheroux and coworkers, 2014) Saccharomyces cerevisiae cells are known to be an excellent dietary source of riboflavin (Bässleret al. 2002) and to possess all enzymes required for riboflavin biosynthesis, which are encoded by the RIB1-7 genes, as reported in Reihl and Stolz (2005).We know little about the enzymes responsible for turnover of FMN and FAD and their subcellular localization, despite the crucial roles of flavin cofactors in metabolism. The formation of holo-flavoproteins, by binding of a flavin prosthetic group to an apoflavoprotein, depends on the availability of FMN and FAD. The homeostasis of riboflavin and flavin prosthetic groups may be altered by some factors, such as defective FMN and/or FAD synthesis, increased FMN and/or FAD catabolism, by different susceptibility of holo- and apo-flavoproteins to proteolytic digestion (Pallotta et al. 1998; Pallotta 2011) and altered mitochondrial transport as was studied by Alex Tzagoloff’s lab. Mitochondria competence to metabolize externally added and endogenous FAD and FMN was investigated by spectroscopically and via HPLC. Thus, two novel yeast mitochondrial enzymatic activities, i.e. FAD pyrophosphatase (diphosphatase; EC 3.6.1.18) and FMN phosphohydrolase (EC 3.1.3.2), which catalyse the reactions FAD +H2O→FMN+AMP and FMN+H2O→ riboflavin + Pi conversion, respectively, were reported (Pallotta (2011). Consequently, besides being the major reservoir of the cellular FMN and FAD, mitochondria represent a site of active metabolism. Therefore yeast seems suitable as model organism to design novel drugs/therapies as new metabolic targeting strategies in human diseases.