Growth signal
The instruction a cell receives to grow and divide. Normal cells wait for it; cancer cells fake it, so they keep dividing whether or not the body wants more of them.
A growth signal begins when a growth factor binds a receptor, passes through a chain of kinases inside the cell (the RAS-MAPK and PI3K-AKT pathways are the two main routes), and ends in the nucleus, where genes for cell division are switched on. Cancers generate the signal without permission by overexpressing receptors, mutating them so they fire on their own, or mutating a downstream relay such as KRAS or BRAF so the message is sent regardless of what happens upstream. 'Sustaining proliferative signalling' is the first of the hallmarks of cancer, and cutting the signal at some point along the chain is what most targeted therapies do.