How fixed is the differentiated state? Lessons from heterokaryons.

The differentiated state is highly stable in vivo. Yet, in response to nuclear transplantation, tissue regeneration or cell fusion, the nuclei of differentiated cells exhibit a remarkable capacity to change. I review here the utility of heterokaryons, multinucleated cell hybrids, in elucidating the mechanisms that establish and maintain the differentiated state and yet allow such plasticity.