Plays a key role in the control of the eukaryotic cell cycle by modulating the centrosome cycle as well as mitotic onset; promotes G2-M transition; and regulates G1 progress and G1-S transition via association with multiple interphase cyclins. Required in higher cells for entry into S-phase and mitosis. Phosphorylates PARVA/actopaxin; APC; AMPH; APC; BARD1; Bcl- xL/BCL2L1; BRCA2; CALD1; CASP8; CDC7; CDC20; CDC25A; CDC25C; CC2D1A; CSNK2 proteins/CKII; FZR1/CDH1; CDK7; CEBPB; CHAMP1; DMD/dystrophin; EEF1 proteins/EF-1; EZH2; KIF11/EG5; EGFR; FANCG; FOS; GFAP; GOLGA2/GM130; GRASP1; UBE2A/hHR6A; HIST1H1 proteins/histone H1; HMGA1; HIVEP3/KRC; LMNA; LMNB; LMNC; LBR; LATS1; MAP1B; MAP4; MARCKS; MCM2; MCM4; MKLP1; MYB; NEFH; NFIC; NPC/nuclear pore complex; PITPNM1/NIR2; NPM1; NCL; NUCKS1; NPM1/numatrin; ORC1; PRKAR2A; EEF1E1/p18; EIF3F/p47; p53/TP53; NONO/p54NRB; PAPOLA; PLEC/plectin; RB1; UL40/R2; RAB4A; RAP1GAP; RCC1; RPS6KB1/S6K1; KHDRBS1/SAM68; ESPL1; SKI; BIRC5/survivin; STIP1; TEX14; beta-tubulins; MAPT/TAU; NEDD1; VIM/vimentin; TK1; FOXO1; RUNX1/AML1; SIRT2 and RUNX2. CDK1/CDC2-cyclin-B controls pronuclear union in interphase fertilized eggs. Essential for early stages of embryonic development. During G2 and early mitosis; CDC25A/B/C-mediated dephosphorylation activates CDK1/cyclin complexes which phosphorylate several substrates that trigger at least centrosome separation; Golgi dynamics; nuclear envelope breakdown and chromosome condensation. Once chromosomes are condensed and aligned at the metaphase plate; CDK1 activity is switched off by WEE1- and PKMYT1-mediated phosphorylation to allow sister chromatid separation; chromosome decondensation; reformation of the nuclear envelope and cytokinesis. Inactivated by PKR/EIF2AK2- and WEE1-mediated phosphorylation upon DNA damage to stop cell cycle and genome replication at the G2 checkpoint thus facilitating DNA repair. Reactivated after successful DNA repair through WIP1-dependent signaling leading to CDC25A/B/C- mediated dephosphorylation and restoring cell cycle progression. In proliferating cells; CDK1-mediated FOXO1 phosphorylation at the G2-M phase represses FOXO1 interaction with 14-3-3 proteins and thereby promotes FOXO1 nuclear accumulation and transcription factor activity; leading to cell death of postmitotic neurons. The phosphorylation of beta-tubulins regulates microtubule dynamics during mitosis. NEDD1 phosphorylation promotes PLK1-mediated NEDD1 phosphorylation and subsequent targeting of the gamma-tubulin ring complex (gTuRC) to the centrosome; an important step for spindle formation. In addition; CC2D1A phosphorylation regulates CC2D1A spindle pole localization and association with SCC1/RAD21 and centriole cohesion during mitosis. The phosphorylation of Bcl- xL/BCL2L1 after prolongated G2 arrest upon DNA damage triggers apoptosis. In contrast; CASP8 phosphorylation during mitosis prevents its activation by proteolysis and subsequent apoptosis. This phosphorylation occurs in cancer cell lines; as well as in primary breast tissues and lymphocytes. EZH2 phosphorylation promotes H3K27me3 maintenance and epigenetic gene silencing. CALD1 phosphorylation promotes Schwann cell migration during peripheral nerve regeneration (By similarity). {ECO:0000250}.