Fully developed cells in the body are generally considered irreversibly developmentally locked, i.e. incapable of being reprogrammed in vivo into entirely different cell types. We found that brief expression of a single transcription factor, a protein in the nucleus that controls gene expression, can convert the identity of fully differentiated, highly specialized mouth cell into an intestinal cells in living worms. Thus, cells that have stopped dividing, though terminally differentiated, can be remodeled adopt the characteristics of a new cell identity without removal of inhibitory factors. Our findings establish a simple model in the worm to investigate how cell context influences forced reprograming of mature cells, which might have relevance to treating human disease via a similar strategy.