IN virtually all eukaryotes, mitosis starts after the completion of DNA synthesis, This orderly process is ensured by the checkpoint mechanism that blocks:the onset of mitosis while DNA is being Synthesized or is damaged, In the fission yeast Schizosaccharomyces pombe, this mechanism involves some rad(+) and hus(+) genes(1-4). However, it is not known how the checkpoint system monitors these events, Recently a multicopy suppressor of a temperature-sensitive DNA polymerase-alpha mutant was isolated, This gene, named cds1(+) (checking DNA synthesis), encodes a typical protein kinase, Here we report that this protein kinase is a key component of the DNA replication-monitoring S/G2 checkpoint system, Our data suggest that its primary role is to monitor DNA synthesis by interacting : with DNA polymerase alpha and send a signal to block the onset of mitosis while DNA synthesis is in progress.