F-box proteins, a critical component of the evolutionary conserved ubiquitin-protein ligase complex SCF (Skp1/Cdc53-Cullin1/F-box), recruit substrates for ubiquitination and consequent degradation through their specific protein-protein interaction domains. Novel human F-box proteins named FBG1, FBG2, FBG3, FBG4 and FBG5 constitute a third subfamily of mammalian F-box proteins. FBG genes are expressed in a limited number of human tissues including kidney, liver, brain and muscle tissues.