Oval cells  which become apparent in the liver after chronic injury  serve as bipotent progenitors for differentiated hepatocytes and cholangiocytes We found that  in the liver of adult transgenic mice in which expression of green fluorescent protein (GFP) is driven by regulatory elements of the nestin gene  the GFP signal marks a subpopulation of small epithelial cells that meet the criteria for oval cells  including morphology  localization  antigenic profile  and reactivity in response to injury