Serine/threonine-protein kinase which is required for checkpoint-mediated cell cycle arrest and activation of DNA repair in response to the presence of DNA damage or unreplicated DNA (PubMed:8497322). May also negatively regulate cell cycle progression during unperturbed cell cycles. Binds to and phosphorylates CDC25. This leads to negative regulation of CDC25 and prevents mitotic entry (PubMed:9278510). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with crb2 (PubMed:9407031, PubMed:14739927, PubMed:22792081). Interacts with rad3 (PubMed:14739927). Interacts with rfp1 (PubMed:17502373). Phosphorylated. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. NIM1 subfamily.