During development, when cells are programmed to perform specific tasks via terminal differentiation, they don’t undergo fate changes and perform their specific tasks throughout life, a process referred to as phenotypic stability. The idea of reprogramming challenged this concept and introduced the notion that the genomic material from any given cell can be reprogrammed to move back in the developmental cascade and become more unrestricted in terms of what other cell types it could generate in the process. It was initially shown that through Somatic Cell Nuclear Transfer (SCNT), one can reprogram the nucleus of any somatic cell by injecting it into oocytes that were experimentally devoid of their own nucleus. Once the oocyte divides and begins the process of developing, pluripotent cells can be isolated to be used for many purposes (e.g., further differentiation into a specific cell type, such as neurons, expansion, etc.).
Over the last decade, induced pluripotent stem cell (iPSC) technology revolutionized the way we now create pluripotent cells from differentiated, somatic cells. By ectopically expressing handful of genes -as low as 2 of them in some cases- differentiated cells can be pushed to express genes that are associated with a more undifferentiated state and eventually transform into “embryonic stem cell like” colonies. Numerous laboratories around the world is utilizing this technology to generate iPSCs for future cell-replacement therapy, studying disease mechanisms, testing drug toxicity and more. However, depending on the cell type that’s being differentiated from the iPSCs for these particular studies, one may not need to generate iPSCs and go straight to the last step of generating a particular cell type (e.g., neurons, beta endocrine cells, etc.) by using direct conversion.
In direct conversion (also referred to as transdifferentiation), the ectopic expression of transcription factors converts somatic cells from one lineage to another without reverting the cell all the way back to a fully undifferentiated state. Direct conversion has been around a lot longer than the iPSC technology. For instance, conversion of fibroblasts into muscle cells by overexpression of MyoD  or conversion of lymphoid cells into macrophages by expressing Pu-1  were among the first demonstrations that a single transcription factor could directly convert one cell type into another. Note that these initial studies achieved conversion from one cell type to another within the same lineage. More recently, using the iPSC approach, there have been studies that achieved direct conversion from one germ layer to that of another, such as fibroblasts to neurons or liver cells. Furthermore, there have been studies that utilized direct conversion in vivo, such as in vivo conversion of exocrine to endocrine pancreas cells , or in vivo conversion of fibroblasts into in functional myocytes in an infarct area . Without a doubt, the success of recent experiments using direct conversion was inspired by Yamanaka’s discovery of iPSC technology, further indicating that the approach has importance beyond just the reprogramming of somatic cells into pluripotent cells.
In direct conversion, one cell type is transformed into another without going through the intermediate pluripotent state (i.e., iPSCs). In specific applications where the pluripotent state is not necessary, this is a welcome change, since the process of creating and maintaining iPSCs are labor intensive and costly. Furthermore, for cell-based replacement therapies, differentiation of cells from iPSCs and transplanting them into recipients carry additional risk factors, such as increased chance of tumor formation, while dealing with pluripotent cells. In the two examples stated above, in which the pancreatic exocrine cells were converted directly into endocrine cells or the resident fibroblasts in the injured heart that were converted into functional cardiomyocytes, there was no need for an intermediate pluripotent state to achieve the goal of transdifferentiation, and the process of conversion took place in vivo. By utilizing the right set of transcription factors, the cells transformed from one lineage into another and achieved the final result of repair and/or restoration of function following disease/injury. Even though, iPSCs would be necessary for many applications, for certain tasks, we can reach the same final aim by using direct conversion without the need for iPSCs. Furthermore, direct lineage conversion could provide important new sources of human cells for creating in vitro disease models and cell-based replacement therapies. While the use of direct conversion gains more popularity, it will be significant to carefully determine the fidelity of reprogramming and to develop methods for robustly and efficiently generating one specific human cell type from another, directly.
 Expression of a single transfected cDNA converts fibroblasts to myoblasts. Davis et al., (1987) Cell, 51, 987-1000
 Stepwise Reprogramming of B Cells into Macrophages. Xie et al., (2004) Cell, 117, 5, 663-676
 In vivo reprogramming of adult pancreatic exocrine cells to beta cells. Zhou et al., (2008) Nature 455, 627-632.
 Heart repair by reprogramming non-myocytes with cardiac transcription factors. Song et al., (2012) Nature, 485, 599–604.