iPS cells are somatic cells that were manipulated to exhibit properties of embryonic stem cells. Introduction of a set of four factors into somatic cells, along with specific culture conditions, alters each cell's epigenetic signature, resetting the cell to a pluripotent ESC-like state. This process is termed "reprogramming." Like ES cells, iPS cells can be differentiated into many different cell types in the lab, and mouse iPS cells have passed even the most stringent tests for pluripotency. iPS cells have been derived from patients affected by a number of diseases, allowing scientists to develop new models of these diseases and screen potential therapeutic agents. iPS cells, therefore, have great potential to contribute to the search for new therapies. Although ES cells remain the gold standard for pluripotency, the scientific community is actively investigating the potential of iPS cells to fulfill many of the research purposes of ESCs. Moreover, if significant safety concerns can be overcome, iPS cells could eventually be valuable to the development of cell-based therapies.