Directed differentiation is primarily applied to pluripotent stem cells (PSCs) of mammalian origin, in particular mouse and human cells for biomedical research applications.[1] Since the discovery of embryonic stem (ES) cells (1981) and induced pluripotent stem (iPS) cells (2006), source material is potentially unlimited.[2][3][6] Historically, embryonic carcinoma (EC) cells have also been used.[7] Fibroblasts or other differentiated cell types have been used for direct reprogramming strategies.[1]