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. Protein modification; protein ubiquitination. Part of the SCF (SKP1-CUL1-F-box) E3 ubiquitin-protein ligase complex SCF(FBXL21) composed of CUL1, SKP1, RBX1 and FBXL21. Interacts with CRY2 (By similarity). Interacts with CRY1. Mainly localizes in the cytosol. Present at low level in the nucleus (By similarity). Expressed in the adenohypophysis, hypothalamus (especially in the suprachiasmatic nucleus or nuclei, SCN) and pineal, all neuroendocrine structures associated with timing and homeostasis.