Forced transdifferentiation is converting a differentiated cell type directly into another mature cell type without going through an intermediate pluripotent stem cell state. Natural transdifferentiation occurs when a gene encoding a major transcription factor is activated in a mature cell. Artificial transdifferentiation can happen by introducing lineage-specific transcription factors or by modifying cells’ chemical environments. For example, mouse heart fibroblasts can be transdifferentiated into cardiomyocytes, which are heart muscle cells, by using viral vectors to introduce genes encoding specific transcription factors. Transdifferentiation can also be induced chemically. For example, dexamethasone converts rat pancreatic exocrine cells into hepatocytes, which are liver cells. Dexamethasone activates the expression of the transcription factor C/EBP β in mature pancreatic cells. C/EBP β is the master switch for differentiation, and when its expression is turned on, it gradually transforms pancreatic cells into hepatocytes. Ongoing research on the transdifferentiation of human fibroblasts into neurons or cardiomyocytes in vitro has shown potential for regenerative medicine.