Cytoplasts can transfer factor(s) that stimulate quiescent fibroblasts to enter S phase.

Cytoplasts prepared from fibroblasts arrested by hydroxyurea were fused with serum-arrested, quiescent fibroblasts. In contrast to unfused mono-nucleated cells in the same culture, a sizable fraction of these cybridoids entered S phase in the absence of extracellular serum stimulation. Because DNA synthesis commenced only after a considerable lag following fusion, it was concluded that the cytoplasts contain factors which initiate the progression of quiescent cells toward S phase.