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.
Choose how you want to understand this
The full explanation.
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 it starts
Most breast cancers begin in one of two places:
- Ductal — in the ducts that carry milk to the nipple. This is the most common starting point. The American Cancer Society puts invasive ductal carcinoma at about 70% to 80% of all breast cancers.
- Lobular — in the lobules that make breast milk. This type grows a bit differently than ductal cancer. It is somewhat more likely to affect 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 but aggressive type. It makes up about 1% to 5% of cases. Instead of forming an obvious lump, it blocks lymph vessels in the skin of the breast. This causes swelling, redness, and a texture that can look like an orange peel. It tends to grow and spread faster than most other types. It needs prompt, aggressive treatment.
Markers that guide treatment
Doctors test every breast cancer for hormone receptors and for a protein called HER2. Hormone-receptor-positive cancers have receptors for estrogen (ER), progesterone (PR), or both. These can often be treated with hormone therapy. That therapy blocks the effect of these hormones on the cancer. HER2-positive cancers have extra amounts of the HER2 protein, which can make cancer grow faster. Drugs that specifically target HER2 have made this type much more treatable than it once was.
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. Hormone therapy and HER2-targeted drugs do not work on it. So it is treated mainly with chemotherapy, and increasingly with immunotherapy for some cases. It tends to grow faster than hormone-receptor-positive cancer. It is more common in younger women and in women with a BRCA1 gene change.
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 combine to point toward different drugs and approaches. This is why a pathology report contains so much detail beyond just the word "cancer."
What to ask your team
Ask what type and subtype of breast cancer you have. Ask whether your cancer is in situ or invasive. Ask whether it is hormone-receptor-positive, HER2-positive, or triple-negative. Ask how your subtype affects your treatment options and outlook.
Grade adds another layer
Along with type and receptor status, doctors also grade breast cancer from 1 to 3, based on how much the cancer cells resemble normal breast cells and how quickly they appear to be dividing. Grade 1 cells look close to normal and tend to grow slowly. Grade 3 cells look very abnormal and tend to grow faster. Grade, type, and receptor status together give a fuller picture than any single piece of information alone.
Sources
Words to know
Tap any term to see what it means.
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.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
Tap a question to save it to your list (kept on this device).
Speak With Trained Specialists & Human Navigators
Cancer Explained provides educational guidance, but does not replace trained specialists, social workers, or your medical team.
Talk to a trained cancer information specialist
Free, confidential assistance from NCI Cancer Information Service via phone, chat, or email.
Contact your oncology team
Locate after-hours contact numbers, portal messages, or urgent triage phone lines.
Find a patient navigator
Get one-on-one help with appointments, logistics, translation, and care coordination.
Find a genetic counselor
Discuss inherited mutation risk, family history, and genetic testing options.
Find an oncology social worker
Access emotional counseling, family support groups, and mental health resources.
Find a financial navigator
Locate copay assistance foundations, grant programs, and lodging/travel support.
Find a clinical-trial specialist
Search matching studies and speak with NCI trial information specialists.
Get urgent help
Immediate emergency guidance for fever (>100.4°F during chemo), severe pain, or shortness of breath.
Help Us Improve This Guide
Did this explanation answer your question and help you determine your next step?
Know someone who needs this?
Plenty of people are looking for something like this and do not know where to start. If this would help a friend or someone you love, send it on — we have written an opening line so you do not have to stare at an empty message. You can change every word of it.
Your message is written and sent in your own email or messaging app — we never see who you send it to, and nothing is added to any list.
Knowledge Check
0 of 4 answered
This self-assessment checks understanding of educational content only. It is not medical advice.
Plain-language explanation of the published sources cited on this page. AI-assisted, source-checked, not clinician-reviewed.
Last updated: 2026-08-18Next planned review: 2027-07-07
How this page was created
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. We do not employ clinicians and do not intend to — our work 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 checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
General education — varies by person. Answers genuinely differ between people. This page explains what commonly varies and points you to your care team 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.
How this page was created
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. We do not employ clinicians and do not intend to — our work 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 checked line by line against the sources listed on it. That confirms the sources support what the page says. 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.
Read more about our editorial process, our use of AI, and our corrections policy.
Spotted a problem? Report an error — a factual mistake, broken or outdated source, confusing wording, or anything that seems unsafe. Please do not include names, medical record numbers, dates of birth, addresses, or other identifying medical information in your report.
After using this page, do you understand what to do next?
Anonymous — we only record the answer, never who gave it.
Related articles
Still have questions?
Educational answers, plain language
Free to print and share