Discovering a stem cell population is the first step to regenerating the tissue they support: "A novel protein marker has been found that identifies rare adult liver stem cells, whose ability to regenerate injured liver tissue has the potential for cell-replacement therapy. ... In the future, this marker will allow for the isolation and expansion of these stem cells, which could then be used to help patients whose livers can no longer repair their own tissue. ... In a healthy liver, proliferation of mature liver and bile-duct lining cells is sufficient to maintain the necessary size and function of the organ. This even works when the liver is confronted with mild and acute injury, but the situation changes when injury to the liver is chronic and severe ... For chronic injury, the liver uses a back-up system that stimulates stem cells to proliferate and eventually differentiate into new liver cells. [Researchers] found that these dual-potential stem cells can be identified and potentially isolated from other liver cells."