Inside every cell in every human body, there's a very special gene, called BRCA1, that regulates how fast cells can divide. A simple mutation within it can be all that's required for us to develop cancer.
BRCA1 -- the short name for 'breast cancer 1, early onset' -- is essentially a tumour suppressor: when it functions correctly, it controls the rate of replication of cells; when it goes wrong, cells can reproduce way faster, which is the fundamental way that cancer tumours grow. This video explains how BRCA1 works -- or doesn't. [TED-Ed]