The exit of some human diploid fibroblasts (WI-38 cells) from the cell cycle in early G1.

The inducibility of DNA synthesis in WI-38 cells synchronized for different parts of G1 upon fusion with HeLa cells in S has been studied. In the heterokaryons 70 to 80% of WI-38 cell nuclei responded to HeLa inducers within one hour after cell fusion. The remainder of WI-38 cell nuclei responded to HeLa inducers after a lag of 3 hours or more. This lag was independent of duration of G1 traversed by WI-38 cells before cell fusion. The results of these studies suggest that 20 to 30% of WI-38 cells retire from the cell cycle in very early G1.