Long-standing biological dogma--that a cell, once committed, can9t alter its fate--has been challenged by recent research. But now scientists are taking a more critical look and finding that some apparent reprogramming of adult cells might instead be a case of cell fusion, prompting calls to establish rigorous standards for demonstrating plasticity. The political stakes are high, because opponents of embryonic stem cell research are using the prospect of reprogramming of adult cells to argue that embryonic stem cells are not needed to develop medical treatments.