Cell cycle
The ordered sequence a cell goes through to copy its DNA and split in two, with checkpoints along the way where it pauses to check for damage. Cancer cells run through the checkpoints.
The phases are G1 (growth), S (DNA copying), G2 (checking) and M (mitosis, the actual division); proteins called cyclins and cyclin-dependent kinases (CDKs) drive the transitions, and tumour suppressors such as RB1 and p53 hold them back until conditions are right. Cancers accelerate the cycle by amplifying cyclin D or losing RB1 and p53, and many drugs act on it: CDK4/6 inhibitors (palbociclib, ribociclib) hold the cell at the G1 checkpoint, while taxanes and vinca alkaloids sabotage mitosis itself. The 'cell-cycle checkpoint' is unrelated to the 'immune checkpoint' of immunotherapy, despite the shared word.