These researchers argue that embryonic stem (ES) cells and induced pluripotent stem (iPS) cells are most likely the same in any aspect that matters: "the pluripotency of ES cells fueled excitement over their use in regenerative medicine. While ethical hurdles associated with the clinical application of human ES cells appeared to have been overcome with the development of methods to create iPS cells, some recent research has suggested that ES and iPS cells have substantial differences in which sets of genes they express. These findings [argue] to the contrary, rekindling hopes that, under the proper circumstances, iPS cells may indeed hold the clinical promise ascribed to them earlier. ... iPS cells are made by introducing three key genes into adult cells. These reprogramming factors push the cells from a mature state to a more flexible embryonic stem cell-like state. Like ES cells, iPS cells can then, in theory, be coaxed to mature into almost any type of cell in the body. Unlike ES cells, iPS cells taken from a patient are not likely to be rejected by that patient's immune system. This difference overcomes a major hurdle in regenerative medicine. ... At this stage, we can't yet prove that they are absolutely identical, but the available technology doesn't reveal differences. ... Some earlier studies have indicated that iPS and ES cells are dissimilar enough to be classified as different cell types. [The researchers] concluded that the differences noted in other studies were not consistent between different laboratories and thus were not likely to be a result of fundamental differences between the cell types."