How can breast cancer develop?

One mutant cell: One normal cell inside your breast mutates (becomes abnormal), and begins to grow. The normal "stop" signal that tell a cell to stop dividing doesn't work, so the cells keep dividing. One cell becomes two, two become four, four become eight, etc. It takes millions of cancer cells to form a tumor you can feel.

Abnormal cells grow: Takes years unless you have a particularly fast growing type associated with brca1 and brcaii genes.