CHOP-10, a dominant-negative member of the C/EBP family of transcription factors, is initially expressed by growth-arrested preadipocytes and sequesters/inactivates C/EBPbeta through heterodimerization with its leucine zipper during 3T3-L1 preadipocyte differentiation. Our previous studies indicated that, FBS leads to the down-regulation of CHOP-10 expression after induction, and releasing C/EBPbeta from inhibitory constraint, allowing the transactivation of C/EBPalpha and PPARgamma genes, transcription factors required for terminal adipocyte differentiation. In the present study, we reported that FBS induced the expression of YY1, which bound to CHOP-10 promoter via two adjacent YY1-binding sites, suppressing its expression. The knock-down of YY1 expression with YY1 siRNA increased the expression of CHOP-10, inhibiting adipocyte differentiation. IGF-1, a growth factor present in greater concentration in FBS, independently induced the expression of YY1, and contributed to the down-regulation of CHOP-10 during the adipocyte differentiation program. Our studies suggested that YY1 can be a new adipocyte differentiation stimulator.