Human epidermal growth factor receptor negative (HER2-negative) breast cancer represents the majority of breast cancer cases worldwide, accounting for roughly 80 percent of all diagnoses. While this classification might sound technical, understanding what it means can help patients and their families navigate treatment options and expectations with greater confidence.
Understanding Human Epidermal Growth Factor Receptor
The human epidermal growth factor receptor, commonly known as HER2, is a protein found on the surface of breast cells that plays an important role in normal cell growth and division. In healthy cells, HER2 helps control how cells grow, divide, and repair themselves. This protein is part of a larger family of receptors called the epidermal growth factor receptor (EGFR) family, which consists of four members: EGFR (also called HER1), HER2, HER3, and HER4. Each of these receptors works together to regulate cell behavior through complex signaling pathways.[2]
When breast cancer is described as HER2-negative, it means that laboratory tests have shown the cancer cells are not producing excessive amounts of the HER2 protein. In other words, the cancer is not driven by an overproduction of this particular growth factor receptor. This is different from HER2-positive breast cancer, where cancer cells make too many copies of the HER2 gene, leading to an overabundance of HER2 proteins on the cell surface, which causes the cancer to grow and spread more aggressively.[11]
How HER2 Status Is Determined
Doctors cannot determine whether breast cancer is HER2-negative or HER2-positive simply by examining a patient or looking at symptoms. Instead, they must analyze a small sample of the tumor tissue obtained either through a biopsy or during surgery. This tissue sample undergoes specialized laboratory testing to measure the amount of HER2 protein present on the cancer cells.[14]
There are several types of tests used to determine HER2 status. One common method is the IHC test (ImmunoHistoChemistry), which uses a chemical dye to stain the HER2 proteins and gives a score ranging from 0 to 3+. A score of 0 to 1+ typically indicates HER2-negative cancer, while a score of 3+ indicates HER2-positive disease. When the score is 2+, it is considered borderline, and additional testing is usually performed. Another test called the FISH test (Fluorescence In Situ Hybridization) uses special labels that attach directly to the HER2 genes in the cells to provide more detailed information about HER2 status.[18]
It’s important to understand that knowing the HER2 status is just one piece of the puzzle when it comes to understanding breast cancer. Doctors also evaluate other receptors on cancer cells, particularly hormone receptors that respond to estrogen and progesterone. These are often referred to as estrogen receptors (ER) and progesterone receptors (PR). The combination of HER2 status and hormone receptor status helps doctors understand the biology of the cancer and determine the most appropriate treatment approach.[14]
The Significance of HER2-Negative Breast Cancer
HER2-negative breast cancer is actually the most common form of the disease. Approximately four out of five breast cancers do not have excessive amounts of HER2 protein. Among these HER2-negative cases, the vast majority—nearly 70 percent—are also positive for hormone receptors, meaning they are classified as HR-positive/HER2-negative. Only about 10 to 15 percent of breast cancers are both hormone receptor-negative and HER2-negative, a subtype known as triple-negative breast cancer.[14]
Generally speaking, HER2-negative breast cancer tends to have a more favorable outlook compared to HER2-positive disease. This is because HER2-positive cancers are often more aggressive due to the excess HER2 protein driving rapid cell growth and division. However, this doesn’t mean HER2-negative cancers are always less serious—many factors influence how any individual cancer will behave, including its stage, grade, hormone receptor status, and the presence of other genetic changes.[14]
Treatment Approaches for HER2-Negative Disease
The HER2 status of breast cancer has important implications for treatment decisions. There are specific medications designed to target and block HER2 proteins, and these therapies work by interfering with the signals that tell cancer cells to grow and divide. However, if a breast cancer is HER2-negative, these HER2-targeted medications will not be effective because there isn’t an overabundance of HER2 protein to target.[14]
For patients with HR-positive/HER2-negative breast cancer, which represents the most common subtype, the standard treatment typically involves endocrine therapy (also called hormone therapy) combined with other medications. Endocrine therapy works by blocking the effects of estrogen or lowering estrogen levels in the body, which can slow or stop the growth of hormone receptor-positive cancer cells. In recent years, new classes of targeted medications have been developed for this subtype, including CDK4/6 inhibitors, which are drugs that help stop cancer cells from dividing.[8][10]
For HR-positive/HER2-negative advanced or metastatic breast cancer, the addition of CDK4/6 inhibitors to standard endocrine therapy has become a common treatment approach. These oral medications work by blocking proteins called cyclin-dependent kinases 4 and 6, which are involved in cell division. By inhibiting these proteins, the drugs can help prevent cancer cells from progressing through their growth cycle. Currently, there are three approved CDK4/6 inhibitors available, and studies have shown they can improve outcomes when combined with endocrine therapy.[10]
When HR-positive/HER2-negative breast cancer no longer responds to endocrine therapy, chemotherapy becomes an essential treatment option. Various chemotherapy regimens may be used depending on the individual patient’s situation, previous treatments, and overall health status. The goal is to find the most effective treatment while minimizing side effects and maintaining quality of life.[12]
The Concept of HER2-Low Breast Cancer
In recent years, scientists and doctors have recognized that the simple classification of breast cancer as either HER2-positive or HER2-negative may not tell the complete story. More than half of the breast cancers traditionally classified as HER2-negative actually have some HER2 proteins on the surface of their cells—just not enough to be considered HER2-positive. This has led to the identification of a new category called HER2-low breast cancer, and in some cases, HER2-ultralow.[18]
The recognition of HER2-low breast cancer is important because new types of medications called antibody-drug conjugates (ADCs) are being developed that may be effective even when HER2 levels are low. These drugs combine an antibody that targets HER2 with a chemotherapy medication, allowing the chemotherapy to be delivered directly to cancer cells that have even small amounts of HER2 protein on their surface. This represents a potential new treatment option for patients whose cancers fall into this HER2-low category.[9]
However, there is still some debate in the medical community about whether HER2-low and HER2-ultralow breast cancers should be considered distinct subtypes. Research is ongoing to better understand these categories and determine how they should influence treatment decisions.[18]
Understanding Hormone Receptor Status in HER2-Negative Cancer
When breast cancer is classified as HER2-negative, doctors look closely at whether the cancer is positive or negative for hormone receptors. This classification is crucial because it determines which treatments will be most effective. The combination of HER2 status and hormone receptor status creates different subtypes of breast cancer, each with its own treatment approach and prognosis.[7]
HR-positive/HER2-negative breast cancer is the most common subtype, accounting for more than two-thirds of all breast cancer cases. For this subtype, endocrine therapy is the backbone of treatment because the cancer cells rely on hormones to grow. Blocking these hormones or their effects can slow or stop cancer growth. In the advanced or metastatic setting, doctors typically use sequential lines of endocrine therapy combined with targeted agents before considering chemotherapy.[7]
Triple-negative breast cancer, which is negative for both hormone receptors and HER2, presents different challenges. Because this subtype doesn’t have the targets that endocrine therapy or HER2-targeted therapy can attack, chemotherapy has traditionally been the main treatment option. However, research is ongoing to identify new therapeutic targets and treatment approaches for triple-negative breast cancer.[13]
Prognosis and Long-Term Outlook
The prognosis for patients with HER2-negative breast cancer varies widely depending on many factors beyond just HER2 status. The stage of the cancer at diagnosis, its grade (how abnormal the cells look under a microscope), hormone receptor status, the patient’s age and overall health, and the presence of specific genetic mutations all play important roles in determining outcomes.[14]
For HR-positive/HER2-negative early-stage breast cancer, the five-year survival rate is generally favorable. However, one characteristic of this subtype is that it can have a longer risk period for recurrence compared to other types of breast cancer. Studies have shown that HR-positive/HER2-negative breast cancer can recur anywhere from 13 to 41 percent of the time over a 20-year period after completing five years of endocrine therapy. This extended risk period means that long-term monitoring and sometimes extended endocrine therapy may be recommended.[7]
For advanced or metastatic HR-positive/HER2-negative breast cancer, while the disease cannot typically be cured, many treatment options are available that can control the cancer for extended periods and maintain quality of life. The challenge is that resistance to treatments can develop over time, requiring changes in treatment strategy. Research has shown that approximately 25 to 50 percent of patients may have de novo (from the beginning) resistance to endocrine therapy, and acquired resistance inevitably develops over time in the metastatic setting.[7]
The Role of Biomarker Testing
Determining HER2 status is part of a broader process called biomarker testing, which looks at various characteristics of cancer cells to help guide treatment decisions. Beyond HER2 and hormone receptors, doctors may test for other biomarkers, particularly in certain situations. For example, genetic testing may be recommended for some patients to look for inherited mutations that increase cancer risk, which can influence treatment decisions and screening recommendations for family members.[6]
The accuracy of HER2 testing is generally very good, but like all medical tests, it is not perfect. If there is any uncertainty about test results, or if the cancer behaves in unexpected ways, doctors may recommend repeat testing or additional types of tests to confirm the HER2 status. It’s always appropriate for patients to ask questions about their test results and understand what they mean for their specific situation.[18]
Living with HER2-Negative Breast Cancer
A breast cancer diagnosis affects every aspect of a person’s life, not just their physical health. While the HER2-negative classification provides important information about the biology of the cancer and guides treatment decisions, it doesn’t define who the patient is as a person. Research has shown that 90 percent of women with breast cancer want people to see them, not just their diagnosis.[17]
Many patients find that connecting with support groups, either in person or online, helps them manage the emotional aspects of diagnosis and treatment. Hearing from others who have walked a similar path can provide valuable insights, practical advice, and emotional support. Support groups create opportunities to ask questions and learn from the experiences of others facing similar challenges.[17]
Talking openly about the diagnosis with family, friends, and healthcare providers is important for managing emotions and getting the support needed throughout treatment. Many patients emphasize the importance of self-advocacy—asking questions, seeking second opinions when appropriate, and being an active participant in treatment decisions. This is particularly important for women of color, who are disproportionately affected by breast cancer and may face additional barriers to care.[17]
Research and Future Directions
The field of breast cancer treatment is continuously evolving, with new discoveries and treatment options emerging regularly. For HER2-negative breast cancer, particularly the HR-positive/HER2-negative subtype, research is focused on understanding mechanisms of resistance to endocrine therapy and developing new targeted treatments that can overcome this resistance.[8]
Scientists are studying the complex signaling pathways that cancer cells use to grow and survive, including pathways involving PI3K (phosphoinositide-3-kinase), AKT, and mTOR proteins. Understanding these pathways may lead to new medications that can work alongside or instead of traditional treatments. Clinical trials are constantly testing new drugs and new combinations of existing drugs to find better ways to treat HER2-negative breast cancer.[9]
The recognition of HER2-low breast cancer as a potentially distinct category has opened new avenues for treatment development. The development of more potent antibody-drug conjugates that can work even when HER2 levels are low represents an important advance that may benefit many patients whose cancers were previously classified simply as HER2-negative.[9]
Research is also examining the role of cell cycle-related genes in cancer development and progression. These genes, which control how cells divide and multiply, may provide new targets for treatment and help doctors better predict which patients are at higher risk for aggressive disease or treatment resistance. Studies are developing prognostic signatures based on gene expression patterns that could help personalize treatment decisions even further.[7]
