The short answer
Breast cancer comes in several types. Most start in the ducts or lobules of the breast. Doctors also classify breast cancer by whether it has hormone receptors (ER/PR) or extra HER2, which guides treatment. Triple-negative breast cancer has none of these markers.
Most breast cancers begin in the milk ducts (ductal) or the milk-making lobules (lobular).
'In situ' means the cancer has not spread beyond where it started; 'invasive' means it has grown into nearby tissue.
Doctors test whether a cancer has hormone receptors (ER and PR) and extra HER2 protein.
These markers guide treatment — for example, hormone therapy for hormone-receptor-positive cancers.
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The full explanation.
The simple version
Breast cancer is not a single disease. Doctors classify it by where it starts and by certain markers on the cancer cells. These details matter because they guide which treatments are likely to work.
Where it starts
Most breast cancers begin in one of two places:
- Ductal — in the ducts that carry milk to the nipple (the most common)
- Lobular — in the lobules that make breast milk
In situ versus invasive
'In situ' means the cancer is still confined to where it began and has not grown into nearby tissue. 'Invasive' means it has spread into surrounding breast tissue. Ductal carcinoma in situ (DCIS) is an early, non-invasive form.
Markers that guide treatment
Doctors test the cancer for hormone receptors (ER and PR) and for extra HER2 protein. Hormone-receptor-positive cancers can be treated with hormone therapy; HER2-positive cancers with HER2-targeted drugs. Triple-negative breast cancer has none of these markers and is treated with other approaches.
Hormone receptor and HER2 status guide which treatments are likely to help.
Words to know
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Common questions
▸Where do most breast cancers start?
Most begin in the ducts that carry milk to the nipple (ductal) or in the lobules that make milk (lobular). Cancers are named partly by where they start.
▸What does 'in situ' versus 'invasive' mean?
'In situ' means the cancer cells are still in the place where they started and have not spread into nearby tissue. 'Invasive' means the cancer has grown beyond that starting point into surrounding breast tissue.
▸What are hormone receptors?
Some breast cancers have receptors for the hormones estrogen (ER) or progesterone (PR). These cancers, called hormone-receptor-positive, can often be treated with hormone therapy that blocks these hormones.
▸What is HER2?
HER2 is a protein that can make some breast cancers grow faster. Cancers with extra HER2 are called HER2-positive and can be treated with drugs that target HER2.
▸What is triple-negative breast cancer?
Triple-negative breast cancer does not have hormone receptors or extra HER2. Because hormone therapy and HER2 drugs do not work on it, it is treated with other approaches such as chemotherapy.
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