The term G0 was introduced over 35 years ago in operational sense, to define cells that do not enter the cycle (incorporate thymidine) for a time period equivalent of two generation times.This term was then adapted by many researchers to define noncycling cells in general or in their particular cell systems, often without any specification. There is still no general consensus as to which attribute of the cell should be considered the marker of Go. Years ago we have noticed that noncycling cells have approximaletely 10 times less RNA than their cycling counterparts (PNAS, 73:2881-2884, 1976) . For many years the low RNA content was considered to be the gold standard of Go state. Cell staining with the metachromatic dye acridine orange or Hoechst/pyronine Y allows one to obtain differential staining of DNA and RNA and easily identify Go cells (e.g see Current Protocols in Cytometry). Low mitochondrial mass and overall mitochondrial transmembrane potential (e.g. measured by rhodamine 123 uptake) was later found to be another marker of Go cells (PNAS, 78: 2383-2387, 1981). With molecular markers of the cell cycle now available the Go state can be defined more specifically than it was possible before. I would consider the cells to be in Go if they still did not yet phosphorylate pRB (prior to the "restriction point"). The antibodies that sense pRB phosphorylation are available and can be used to identify such cells (e.g. Juan et al., Exp. Cell Res, 239: 104-110, 1998). One can also define Go cells as the postmitotic cells that did not yet express D type cyclins (Cytometry, 25: 1-13, 1996). It should be noted, however, that the cell ability to enter genuine Go characterizes normal cells. Most tumor type cells have either defective pRB pathway or constant mitogenic signaling upstream of pRB and therefore by-pass Go. Zbigniew Darzynkiewicz
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