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 whole article, with every detail and caution.
This page is for people who want to understand the different kinds of breast cancer. It explains where breast cancer starts, the markers doctors test for, and why the type matters.
The simple version
Breast cancer is not a single disease. Doctors classify it by where it starts in the breast and by certain markers on the cancer cells. These details matter because they guide which treatments are likely to work.
Where breast cancer starts
Most breast cancers begin in one of two places:
- Ductal: in the ducts that carry milk to the nipple. NCI calls invasive ductal carcinoma the most common breast cancer diagnosis.
- Lobular: in the lobules that make breast milk. NCI says invasive lobular carcinoma grows more slowly, is less common, and is more often found in both breasts.
In situ versus invasive
'In situ' means the cancer cells are still confined to where they began. They have not grown into nearby tissue. Ductal carcinoma in situ, known as DCIS, is an early, non-invasive form. Doctors sometimes call it stage 0 breast cancer. 'Invasive' means the cancer has grown beyond that starting point into surrounding breast tissue. This is what most people mean when they say breast cancer.
Inflammatory breast cancer
Inflammatory breast cancer is a rare form of breast cancer. NCI says it has a high risk of recurrence. The cancer cells block lymph vessels, which makes the breast look swollen and red, or inflamed. If you notice these changes, call your care team promptly.
Markers that guide treatment
NCI says breast cancer subtypes are defined by whether the cancer has hormone receptors, HER2 protein, or other biomarkers. Hormone-receptor-positive cancers have receptors for estrogen (ER), progesterone (PR), or both. NCI says hormone therapy helps lower the risk of recurrence of certain types of breast cancer. HER2-positive cancers have extra amounts of the HER2 protein, which can make cancer grow faster. Targeted drugs can treat cancers with this feature.
Hormone receptor and HER2 status guide which treatments are likely to help.
Triple-negative breast cancer
Triple-negative breast cancer lacks hormone receptors and extra HER2. NCI says it has a higher risk of recurrence than most other forms. NCI says its treatments may include surgery, chemotherapy, and more, and that immunotherapy may be used. Ask your team what is right for you.
Why all of this matters together
Two people with a similar-sized tumor can need very different treatment plans. The type, whether it is in situ or invasive, and the hormone receptor and HER2 status all help guide which treatments are used. This is why a pathology report contains so much detail beyond just the word "cancer."
What to ask your team
- What type and subtype of breast cancer do I have?
- Is my cancer in situ or invasive?
- Is it hormone-receptor-positive, HER2-positive, or triple-negative?
- How does my subtype affect my treatment options and outlook?
Sources
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. NCI says its treatments may include surgery, chemotherapy, and more.
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Last updated: 2026-10-16Next planned review: 2027-07-07
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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