Introduction to the cell | Cells | High school biology | Khan Academy

Cancer doesn't start as a lump. It starts with a single line of DNA going off script. 🔬 Cancer is what happens when abnormal cells grow out of control, and it begins deep inside the cell. Your DNA holds the genes that code for everything a cell does, including two critical instructions: when to divide, and when to undergo apoptosis (controlled, programmed cell death). Normally these are tightly regulated. But when certain genes mutate, that order breaks down and cells start dividing unchecked.