Summary. Transcription of p21 was activated in hepatitis C virus (HCV) Core-expressing HepG2 cells where its upstream p53 was stabilized. However, this effect was not absolutely required for the activation of p21 by Core, as demonstrated in Hep3B cells. In addition, an opposite effect on the transcription of p21 was observed in NIH3T3 and primary hepatocytes, where p53 was not decreased by Core. To explain the p53-independent regulation of p21 by Core, we identified a Core-responsive element between positions −74 and −83 of the p21 promoter, exactly overlapped with a tumour growth factor β (TGF-β)/butyrate responsive element. Furthermore, we demonstrated that Core could activate the p21 through the element by stimulating a butyrate pathway, whereas this was inhibited through a TGF-β pathway. The opposing effects of Core protein on the transcription of p21 might be important in understanding the progression of hepatic disease in HCV-positive patients.