Subtypes of Breast Cancer

Breast cancer varies in how it grows and responds to treatment. Some tumours grow quickly, while others develop more slowly. To better understand how a tumour may behave and how to treat it, healthcare teams classify breast cancer based on biomarkers—specific biological features found through lab testing.

The two key biomarkers tested when diagnosing breast cancer are hormone receptor (HR) status, and human epidermal growth factor receptor 2 (HER2) status. Healthcare teams use these, along with tumour stage and grade, to guide treatment decisions. In addition to these biomarkers, breast cancer can be grouped into molecular subtypes.

Note: A person’s subtype can change over time. In 20–30% of cases, the subtype may change if it recurs or spreads.

What Is Hormone Receptor (HR) Status?
Some breast cancer cells have receptors (proteins in or on cells) that attach to the hormones estrogen or progesterone. These hormones can help the cancer cells grow.

Breast cancer cells may have a combination of these receptors:

• Estrogen receptor (ER) positive: Has estrogen receptors
• Progesterone receptor (PR) positive: Has progesterone receptors
• Estrogen receptor (ER) positive, Progesterone receptor (PR) positive: Has both receptors

When breast cancer cells have one or both of these receptors, it is known as hormone receptor (HR) positive. If the breast cancer cells have neither of these receptors, it is known as hormone receptor (HR) negative.

Why Is HR Status Important?

• Blocking these hormones with treatment can slow or stop cancer growth.
• HR positive cancers grow more slowly and respond well to hormone therapy.
• HR negative cancers grow faster, do not respond to hormone therapy, and often require chemotherapy.
• About 75% of breast cancers have at least one hormone receptor, with HR positive cancers being more common in older adults.

What Is HER2 Status?
HER2 is a protein found on breast cells that helps control cell growth. The HER2 gene controls how much HER2 protein a cell makes.

• Normal breast cells have two copies of the HER2 gene and normal HER2 protein levels.
• HER2 positive breast cancer has extra copies of the HER2 gene, leading to extra HER2 protein and uncontrolled growth.

Why Is HER2 Status Important?

• Doctors test HER2 status on tumour tissue to determine treatment options.

• HER2 postive cancers grow faster than HER2- ones. They respond well to targeted therapies like trastuzumab (Herceptin) and pertuzumab (Perjeta).
• HER2 status can change over time, especially after treatment, recurrence, or metastasis, requiring repeat testing.

Molecular Subtypes of Invasive Breast Cancer
In addition to clinical classifications based on HR and HER2 status, breast cancer can be further divided into molecular subtypes. These subtypes are based on genes and proteins in the cells, helping predict how the cancer may grow and respond to treatment.

The main molecular subtypes include:

• Luminal A (HR positive/HER2 negative): Slow-growing, low levels of Ki-67 (a marker of cell growth)
• Luminal B (HR positive/HER2 positive or HER2 negative): Faster-growing, higher Ki-67 levels
• HER2-Enriched (HER2 positive): HER2 positive, HR negative (ER negative, PR negative)
• Triple-Negative/Basal-Like (TNBC): ER negative, PR negative, and HER2 negative. More common in younger people, Black individuals, and BRCA1 carriers

Why are Molecular Subtypes Important?
Molecular subtypes help predict tumour behavior and guide treatment decisions. While not always included in pathology reports, they are essential in precision medicine and the development of targeted therapies.

Each subtype has distinct characteristics and treatment approaches. The following pages explore these subtypes in more detail and their impact on patient care.

Medical Review by Roochi Arora, MD, FRCPC, August 2025

Our Stories

More Stories

Tell Us your Story