The objective of this study is to investigate whether F-box only protein 9 (FBXO9), an ubiquitination E3 ligase, has a functional role in adipocyte differentiation. Expression of FBXO9 was compared between obese mice and control lean mice using real-time PCR. Also, expression pattern of FBXO9 was monitored during 3T3-L1 adipocyte differentiation. FBXO9 was highly expressed in obese mice, and increased in the early stages of adipogenesis. To verify a functional role of FBXO9 in adipogenesis, FBXO9 was knocked down using transfection of siRNAs against FBXO9 into 3T3-L1 cells during the induction of adipogenesis. Knockdown of FBXO9 in early stage of adipogenesis almost completely inhibited adipogenesis, and CCAAT/enhancer binding protein beta (C/EBP beta) levels were significantly reduced. However, the cells stably expressing C/EBP beta were fairly differentiated into adipocytes in the FBXO9 knockdown conditidn. These results suggest that FBXO9 is required for adipocyte differentiation, and C/EBP beta plays a role in the effect of FBXO9 on adipogenesis. (C) 2013 Elsevier Inc. All rights reserved.