Cells are the building blocks of every living thing — from tomatoes to ladybugs to salmon to people. The instructions that tell a cell what to do are in genes within the center of the cell. Those genes are made of DNA (deoxyribonucleic acid). DNA can change or be damaged over time. Some DNA changes are harmless, but others can cause disease. Cancer cells are “born” when abnormal changes in DNA tell cells to grow faster and behave differently than they should. As these cancer cells multiply to form a tumor, they continue to change — becoming more and more different from each other.
As a cancer grows, new and different types of breast cancer cells are created within that same cancer. The mixture of cells that builds up over time becomes more and more complex. So even though every cell of a cancer is related to the same original "parent" cell, all the cells that make up a cancer are not the same. The idea that different kinds of cells make up one cancer is called "tumor heterogeneity."
By the time a breast cancer tumor is one centimeter (less than half an inch), the millions of cells that make up the lump are very different from each other. And each cancer has its own genetic identity, or fingerprint, created by the DNA in its cells. So two people with breast cancer who are the same age, height, weight, and ethnicity, and who have similar medical histories, almost surely have two very different cancers. The only thing the cancers have in common is that they started from a breast tissue cell.