Colony survival of irradiated cells may not always correlate well with the extent of checkpoint arrest in mutants with a partial checkpoint defects (see e.g. Ref. 71). Consequently, we directly analyzed the effect of mutated versions of Rad17p on checkpoint arrest following irradiation of synchronized cells. We placed cells that had been synchronized as large budded cells with hydroxyurea on solid medium and determined the fraction of microcolonies with more than 2 cell bodies as a function of time after ␥ or UV irradiation (Fig. 5B and **data** not  shown). This method measures the extent of arrest at checkpoints downstream of S before entry into the next S-phase and provides a reliable estimate of the prevailing M-phase arrest (13,71). As predicted, [F121L]Rad17p was equally active as the wild-type protein whereas [F121A]Rad17p was inactive and not different from Rad17-1p or empty vector controls. However, we reproducibly detected abbreviated arrest for [F121D]Rad17p and thus demonstrated a partial defect in activity (Fig. 5B, Table I).