This term describes how mature the breast cancer cells are compared to normal breast cells. Well-differentiated tumor cells that are mature look a lot like normal breast cells and tend to grow slowly. Undifferentiated, or poorly differentiated, tumor cells do not look or work like normal cells. They grow quickly and have a tendency to spread.
For more information, see Breastcancer.org's Rate of Cell Growth page.
Audio contributed by: Whoopi Goldberg