There isn’t one single number, because breast cancer is classified in multiple ways. By where the cancer starts and how it behaves under a microscope, there are roughly a dozen recognized types. By molecular profile, which drives most treatment decisions today, there are at least four major subtypes, with newer classifications pushing that number higher. Understanding both layers matters, because two cancers that look identical under a microscope can behave very differently depending on their molecular makeup.
The Two Most Common Types
About 70 to 80% of all breast cancers fall into one of two categories based on where they originate in the breast.
Invasive ductal carcinoma starts in a milk duct and then spreads into surrounding breast tissue. It’s the most common type by a wide margin, accounting for the majority of diagnoses in both women and men. In men, it makes up roughly 85% of cases.
Invasive lobular carcinoma begins in the lobules, the small glands that produce milk. It’s the second most common type in women, representing about 15% of cases. In men, it’s rare (around 1 to 2%) because male breast tissue contains ducts but very few lobules.
Non-Invasive Breast Cancer
Not all breast cancer has spread beyond its point of origin. Ductal carcinoma in situ (DCIS) is a non-invasive form where abnormal cells line a breast duct but haven’t pushed into surrounding tissue. It’s sometimes called stage 0 breast cancer. DCIS is highly treatable, but it does carry a risk of eventually becoming invasive if left untreated, which is why it’s still taken seriously and monitored or treated at diagnosis.
Less Common Histological Types
Beyond the big two, several rarer forms account for the remaining cases. These include mucinous carcinoma, papillary carcinoma, tubular carcinoma, and medullary carcinoma, among others. Each has distinct features under a microscope and can carry different prognoses, but they’re uncommon enough that most people will never encounter them.
Two rare types worth knowing about:
- Inflammatory breast cancer doesn’t form a typical lump. Instead, cancer cells block lymph vessels in the skin, making the breast look swollen, red, and pitted (sometimes compared to an orange peel). It’s aggressive and accounts for a small fraction of cases, but its unusual presentation means it can be mistaken for an infection.
- Phyllodes tumors make up less than 1% of all breast tumors. They grow from the connective tissue of the breast rather than the ducts or lobules. Most are benign (35 to 64%), with the rest classified as borderline or malignant. They typically show up as a rapidly growing lump in women in their 40s or 50s and are often mistaken for a common, harmless growth called a fibroadenoma on imaging. Death from phyllodes tumors is rare, occurring in about 2% of cases, and only those with uniformly aggressive features tend to pose a serious threat.
The Four Molecular Subtypes
The classification system that matters most for treatment decisions is based on what’s happening on the surface of the cancer cells. Doctors test every breast cancer for three key receptors: estrogen receptors (ER), progesterone receptors (PR), and a protein called HER2. The combination of these results sorts cancers into four molecular subtypes, each with different behavior and treatment options.
Luminal A (HR-positive, HER2-negative) is by far the most common, making up about 70% of all female breast cancers. These tumors have hormone receptors but lack excess HER2 protein. They tend to grow more slowly and respond well to hormone-blocking therapies.
Luminal B (HR-positive, HER2-positive) accounts for roughly 9% of cases. Like Luminal A, these cancers are fueled by hormones, but they also overproduce HER2. They tend to grow faster than Luminal A tumors and are treated with both hormone therapy and HER2-targeted drugs.
HER2-enriched (HR-negative, HER2-positive) makes up about 4% of cases. These cancers don’t respond to hormone therapy but do respond to treatments that target the HER2 protein. Before HER2-targeted drugs existed, this subtype carried a worse prognosis. Those treatments have dramatically improved outcomes.
Triple-negative (HR-negative, HER2-negative) accounts for roughly 11% of female breast cancers. With no hormone receptors and no HER2 overproduction, there are fewer targeted treatment options. Chemotherapy has traditionally been the backbone of treatment, though newer approaches including immunotherapy are expanding the options.
Triple-Negative Is Not One Disease
Triple-negative breast cancer (TNBC) is itself a collection of distinct diseases lumped under one label. Gene expression studies have identified at least four molecular subtypes within TNBC, each with different biology and different responses to treatment.
Two of these subtypes are “basal-like,” meaning the cancer cells resemble the basal cells that line the breast ducts. Between 50 and 86% of triple-negative cancers show basal-like features, which is why the terms are sometimes used interchangeably, but they shouldn’t be. The other major subtypes within TNBC include a “luminal androgen receptor” type (driven by androgen rather than estrogen, making up 15 to 25% of TNBC), a mesenchymal type (15 to 20%), and immune-related subtypes that differ in how much the immune system is engaged with the tumor. These distinctions increasingly guide treatment. For example, one of the basal-like subtypes responds better to chemotherapy given before surgery, while the androgen-driven subtype may respond to drugs that block androgen signaling.
HER2-Low: A Newer Classification
Until recently, HER2 testing produced a simple yes-or-no answer: either a cancer overproduced HER2 or it didn’t. Cancers that tested negative were all treated the same way regardless of whether they had zero HER2 or just a small amount. That changed when a newer drug, an antibody-drug conjugate, proved effective in cancers with low levels of HER2 that wouldn’t have qualified as “HER2-positive” under the old system.
This created a new category called HER2-low, which captures cancers that show some HER2 on their surface but not enough to be considered HER2-positive. It’s a significant reclassification because a large proportion of cancers previously labeled simply “HER2-negative” actually fall into this HER2-low range, opening a treatment option that didn’t exist for them before. Cancers with truly no detectable HER2 are now sometimes called “HER2-zero” to distinguish them.
Why Classification Keeps Evolving
Thirty years ago, breast cancer was treated largely as one disease. Today, the combination of where a tumor starts, what it looks like under a microscope, and which receptors and genes are active creates a detailed profile that shapes every treatment decision. A hormone-receptor-positive invasive ductal carcinoma and a triple-negative invasive ductal carcinoma share a name but are, for practical purposes, different diseases with different treatments and different outlooks.
If you’re trying to put a single number on it, there are at least 4 major molecular subtypes, roughly a dozen histological types, and growing subcategories within each. The number keeps increasing as genetic profiling reveals more distinctions that actually change how cancers are treated. For any individual diagnosis, the specific receptor status and molecular profile matter far more than the broad category name.