Decreased sensitivity to hydroxyurea and to [3H]thymidine suicide in the middle of the S phase.

Pluripotent haemopoietic stem cells (CFUs) move synchronously through the cell cycle in hydroxyurea-treated mice in a cohort 1--2 hr broad. Ten to fifteen hours after hydroxyurea they pass through S phase. DNA synthesis appears to be depressed 5--10 times when the cells are in the middle part of the S phase but does not seem to be completely interrupted. High concentrations of [3H]thymidine must be used for 'suicide' in order to achieve lethality for the cells with depressed DNA synthesis. At the time when DNA synthesis is depressed, the sensitivity of the cells to hydroxyurea also decreases. This may lead to a significant underestimation of the S phase fraction by the hydroxyurea method, because CFUs with low DNA synthesis rate are resistant to hydroxyurea although being in S phase.