Required for both DNA replication and mitosis (PubMed:11125146, PubMed:22581055, PubMed:21856198, PubMed:14993212, PubMed:26842564). DNA replication licensing factor, required for pre-replication complex assembly. Cooperates with CDC6 and the origin recognition complex (ORC) during G1 phase of the cell cycle to promote the loading of the mini-chromosome maintenance (MCM) complex onto DNA to generate pre-replication complexes (pre-RC)(PubMed:14672932). Required also for mitosis by promoting stable kinetochore-microtubule attachments (PubMed:22581055). Potential oncogene (By similarity). Interacts with GMNN; the interaction inhibits binding of the MCM complex to origins of replication (PubMed:11125146, PubMed:14672932, PubMed:14993212, PubMed:15257290). Interacts with MCM6 (By similarity). Interacts with CDC6; are mutually dependent on one another for loading MCM complexes onto chromatin (PubMed:14672932). Interacts with PCNA (PubMed:16861906). Interacts with LRWD1 during G1 phase and during mitosis (PubMed:22645314). Interacts with NDC80 subunit of the NDC80 complex; leading to kinetochore localization (PubMed:22581055). Interacts with GRWD1; origin binding of GRWD1 is dependent on CDT1 (PubMed:25990725). Interacts with KAT7 (PubMed:18832067). Interacts with ubiquitin-binding protein FAF1; the interaction is likely to promote CDT1 degradation (PubMed:26842564). Transiently localizes to kinetochores during prometaphase and metaphase. Present during G1 and early S phase of the cell cycle. Degraded during the late S, G2, and M phases. Induced by E2F transcription factors (PubMed:14990995). The PIP-box K+4 motif mediates both the interaction with PCNA and the recruitment of the DCX(DTL) complex: while the PIP-box interacts with PCNA, the presence of the K+4 submotif, recruits the DCX(DTL) complex, leading to its ubiquitination. Two independent E3 ubiquitin ligase complexes, SCF(SKP2) and the DCX(DTL) complex, mediated CDT1 degradation in S phase. Ubiquitinated by the DCX(DTL) complex, in response to DNA damage, leading to its degradation. Ubiquitination by the DCX(DTL) complex is necessary to ensure proper cell cycle regulation and is PCNA-dependent: interacts with PCNA via its PIP-box, while the presence of the containing the 'K+4' motif in the PIP box, recruit the DCX(DTL) complex, leading to its degradation. Phosphorylation at Thr-29 by CDK2 targets CDT1 for ubiquitination by SCF(SKP2) E3 ubiquitin ligase and subsequent degradation (PubMed:14993212). The interaction with GMNN protects it against ubiquitination. Deubiquitinated by USP37 (PubMed:27296872). Phosphorylation by cyclin A-dependent kinases at Thr-29 targets CDT1 for ubiquitynation by SCF(SKP2) E3 ubiquitin ligase and subsequent degradation (PubMed:14993212). Phosphorylated at Thr-29 by MAPK8/JNK1, which blocks replication licensing in response to stress (PubMed:21856198). Binding to GMNN is not affected by phosphorylation. The disease is caused by variants affecting the gene represented in this entry. Belongs to the Cdt1 family.