The concept of a progenitor cell is ifficult to define. Like stem cells, progenitor cells have a capacity to differentiate into a specific type of cell. In contrast to stem cells, however, they are already far more specific than stem cells: they are pushed to differentiate into their "target" cell. Controversy about the exact definition remains and the concept is still evolving, since it is only a decade old. Despite the difficulty of defining progenitors, the term is frequently used in research. Thus the importance of progenitors cannot be ignored.