The three types of breast cancer are defined by what fuels the tumor’s growth: hormone receptor-positive (HR+), HER2-positive, and triple-negative. These aren’t based on where the cancer starts or what it looks like under a microscope. They’re based on which proteins sit on the surface of the cancer cells, and they determine both how aggressive the cancer is and which treatments will work against it.
How Your Subtype Is Determined
After a biopsy, a pathologist tests the tumor tissue for three specific receptors: estrogen receptors, progesterone receptors, and a protein called HER2. The primary method is immunohistochemistry (IHC), a staining technique applied to thin slices of tissue. For HER2 specifically, a second test called FISH (fluorescence in situ hybridization) is sometimes used to confirm results by looking directly at whether the HER2 gene has extra copies.
The combination of these results places the cancer into one of three categories. Each one behaves differently, responds to different treatments, and carries a different outlook.
Hormone Receptor-Positive Breast Cancer
Hormone receptor-positive breast cancer is the most common type, making up roughly 70% of all diagnoses. These tumors have receptors for estrogen, progesterone, or both on their cell surfaces. The hormones your body naturally produces attach to these receptors and signal the cancer cells to grow. In practical terms, the tumor is feeding on your hormones.
Because this type has a clear growth signal that can be blocked, it tends to have the best prognosis. The five-year relative survival rate for HR-positive, HER2-negative breast cancer is 95.6%, according to SEER data from the National Cancer Institute.
Treatment centers on hormone therapy, which works by either lowering hormone levels in your body or blocking the receptors so hormones can’t attach. This therapy typically continues for five years after surgery, and it substantially reduces the risk of the cancer coming back or spreading. Some patients also receive chemotherapy depending on the tumor’s size, grade, and other factors.
Luminal A vs. Luminal B
Within the HR-positive category, researchers distinguish between two molecular subtypes. Luminal A tumors are low grade, slow growing, and have the best prognosis of any breast cancer. They have strong hormone receptor expression and low levels of a cell-division marker called Ki-67 (below 20%). Luminal B tumors, which make up 10 to 20% of hormone receptor-positive cancers, grow faster and are more likely to need chemotherapy in addition to hormone therapy. They have higher Ki-67 levels (above 20%), are often higher grade, and carry a worse prognosis than Luminal A, though still better than the other two main types.
HER2-Positive Breast Cancer
About 15 to 20% of breast cancers are HER2-positive. In these tumors, the gene that makes the HER2 protein has gone into overdrive, producing far too many copies of the protein on the cell surface. HER2 is the strongest growth-signaling protein in its family. When it’s overexpressed, it sends powerful signals that tell cancer cells to divide rapidly and resist normal cell death. The result is a faster-growing, more aggressive cancer with higher rates of spreading to other parts of the body.
Before targeted therapies existed, HER2-positive breast cancer had one of the worst outcomes. That changed dramatically with drugs that specifically latch onto the HER2 protein and shut down its signaling. The standard treatment is one year of HER2-targeted therapy, often combined with chemotherapy. If the tumor also has hormone receptors (HR-positive/HER2-positive), hormone therapy is added as well.
Today, the five-year survival rate for HR-positive/HER2-positive breast cancer is 91.8%. For HR-negative/HER2-positive, it’s 86.5%. Both represent a massive improvement from the era before targeted treatments were available.
Triple-Negative Breast Cancer
Triple-negative breast cancer (TNBC) tests negative for all three markers: estrogen receptors, progesterone receptors, and HER2. With less than 1% expression of hormone receptors and no HER2 overexpression, there is no obvious “off switch” to target. This makes it the most difficult subtype to treat and the one with the poorest prognosis, with a five-year relative survival rate of 78.4%.
TNBC accounts for about 10 to 15% of breast cancers. The cells tend to be moderate to high grade and highly proliferative, meaning they divide quickly. Because hormone therapy and HER2-targeted drugs are ineffective against it, chemotherapy has historically been the backbone of treatment. Combination chemotherapy regimens are standard, and adding platinum-based drugs to traditional regimens has shown promising results in clinical studies.
TNBC also has a distinct risk profile. Black women in the United States have significantly higher rates of triple-negative breast cancer than white women. Several reproductive factors help explain this disparity: earlier age at first menstrual period, earlier age at first birth, and lower rates of breastfeeding all increase TNBC risk, and these patterns are more common among Black women. Carriers of BRCA1 gene mutations also face elevated risk. Interestingly, having a first child at age 30 or older actually decreases TNBC risk by more than 20% compared to having a first child before age 20, which is the opposite of what’s seen with breast cancer overall.
How HER2-Low Is Changing the Picture
Until recently, HER2 results were binary: positive or negative. That’s shifting. Pathologists now recognize that many tumors previously classified as “HER2-negative” actually produce small amounts of the HER2 protein. These are called HER2-low, and an even finer distinction, HER2-ultralow, captures tumors with barely detectable levels.
This matters because a newer class of drug, an antibody-drug conjugate, has shown survival benefits in patients with metastatic breast cancer that has even minimal HER2 expression. Both the FDA and the European Medicines Agency have approved this therapy for patients with HER2-low or HER2-ultralow metastatic breast cancer. As a result, pathology reports are now being updated to explicitly distinguish between truly zero HER2 expression and these very low levels, since the difference can open up a treatment option that didn’t previously exist.
For patients with hormone receptor-positive or triple-negative disease, this means the HER2 portion of your pathology report carries more nuance than it used to. A result that would have simply read “HER2-negative” a few years ago may now specify whether you fall into the HER2-low or HER2-ultralow range, which could matter if the cancer ever becomes metastatic.
Survival Rates at a Glance
Five-year relative survival rates across all stages, based on the most recent SEER data (2015 to 2021):
- HR-positive/HER2-negative: 95.6%
- HR-positive/HER2-positive: 91.8%
- HR-negative/HER2-positive: 86.5%
- Triple-negative: 78.4%
These numbers reflect all stages combined. Early-stage cancers of any subtype have substantially higher survival rates than those diagnosed after spreading. The overall five-year relative survival for breast cancer across all subtypes is 91.7%.