The generation of human embryonic stem cell (hESC) lines (Shamblott et al. 1998; Thomson et al. 1998) opened up the exciting possibility of using these pluripotent stem cells to study the function of differentiated derivative cell types. However, due to the technical and ethical difficulties, it is not feasible to produce a large number of such lines or derive them from patients with diseases, limiting their use to studies of normal cellular function, or to introduction of known engineered genetic changes.
