Slow-cycling progenitor cells are generally resistant to the transfection of SV40 large T antigen for the establishment of cell line. The transfection prevents the senescence. You have to bear in mind that quiescent undifferentiated cells become proliferative due to the drastic change of the surrounding microenvironment. For instance, while FGF-1 induces the proliferative conditions of hematopoietic stem cells (HSCs) near the sinusoid-megakaryocyte niche, TGF-beta is responsible for the dormant status near the vascular or osteogenic niche (In the Current Issue of Nature Medicine ).