Substrate-recognition component of the SCF(FBXL21) E3 ubiquitin ligase complex involved in circadian rhythm function. Plays a key role in the maintenance of both the speed and the robustness of the circadian clock oscillation. The SCF(FBXL21) complex mainly acts in the cytosol and mediates ubiquitination of CRY proteins (CRY1 and CRY2), leading to CRY proteins stabilization. The SCF(FBXL21) complex counteracts the activity of the SCF(FBXL3) complex and protects CRY proteins from degradation. Involved in the hypothalamic suprachiasmatic nucleus (SCN) clock regulating temporal organization of the daily activities (By similarity). . This is the function of Ensembl gene identifier ENSG00000164616 (FBXL21P, F-box and leucine rich repeat protein 21, pseudogene).