FBL3B is a 434 amino acid protein encoded by the human gene FBXL21. FBL3B contains one 40 amino acid F-box region, making it a member of the F-box family. FBL3B also contains three LRR (leucine-rich) repeats. F-box proteins are critical components of the SCF (Skp1-CUL-1-F-box protein) type E3 ubiquitin ligase complex and are involved in substrate recognition and recruitment for ubiquitination. F-box proteins are members of a large family that regulates cell cycle, immune response, signaling cascades and developmental programs by targeting proteins, such as cyclins, cyclin-dependent kinase inhibitors, IkB-a and b-catenin, for degradation by the proteasome after ubiquitination. FBL3B is a substrate-recognition component of the SCF complex that interacts with Skp1 p19 and CUL-1. FBL3B is also associated with expression and regulation of circadian and cryptochrome proteins.