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Exposure of a wide variety of cells to ionizing (X- or γ-) irradiation results in a division delay which may have several components including a G₁ block, a G₂ arrest or an S phase delay. The G₁ arrest is absent in many cell lines, and the S phase delay is typically seen following relatively high doses (> 5 Gy). In contrast, the G₂ arrest is seen in virtually all eukaryotic cells and occurs following high and low doses, even under 1 Gy. The mechanism underlying the G₂ arrest may involve suppression of cyclin B1 mRNA and/or protein in some cell lines and tyrosine phosphorylation of p34^cdc2 in others. Similar mechanisms are likely to be operative in the G₂ arrest induced by various chemotherapeutic agents including nitrogen mustard and etoposide. The upstream signal transduction pathways involved in the G₂ arrest following ionizing radiation remain obscure in mammalian cells; however, in the budding yeast the rad9 …