Triple-negative breast cancer (TNBC) is any breast cancer that either lacks or shows low levels of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) overexpression and/or gene amplification (i.e. the tumor is negative on all three tests, giving it the name triple-negative). Triple-negative is sometimes used as a surrogate term for basal-like.

Triple-negative breast cancer comprises 15–20% of all breast cancer cases and affects more young women or women with a mutation in the BRCA1 gene than other breast cancers. Triple-negative breast cancers comprise a very heterogeneous group of cancers. TNBC is the most challenging breast cancer type to treat. Hormone therapy that is used for other breast cancers does not work for TNBC. In its early stages, the cancer is typically treated through surgery, radiation and chemotherapy. In later stages where surgery is not possible or the cancer has spread from the initial localised area, treatment is limited to chemotherapy and in some cases further targeted therapy.

Triple-negative breast cancers have a relapse pattern that is very different from hormone-positive breast cancers where the risk of relapse is much higher for the first 3–5 years, but drops sharply and substantially below that of hormone-positive breast cancers afterwards.

The overall proportion of TNBC is very similar in all age groups. Younger women have a higher rate of basal or BRCA related TNBC, while older women have a higher proportion of apocrine, normal-like and rare subtypes including neuroendocrine TNBC.

A study in the US has shown that, among younger women, African American and Hispanic women have a higher risk of TNBC, with African Americans facing worse prognosis when diagnosed later than other ethnic groups.

One known risk factor for triple-negative breast cancer is germline mutations. These are alterations within the heritable lineage that is being passed down to the offspring. Due to their high disposition for cancers of the breast, ovaries, pancreas, and prostate, the BRCA1 and BRCA2 genes were identified as high risk for triple-negative. Changes or mutations in 19p13.1 and MDM4 loci have also been associated with triple-negative breast cancer, but not other forms of breast cancer. Thus, triple-negative tumors may be distinguished from other breast cancer subtypes by a unique pattern of common and rare germline alterations.

In 2009, a case-control study of 187 triple-negative breast cancer patients described a 2.5 increased risk for triple-negative breast cancer in women who used oral contraceptives (OCs) for more than one year, compared to women who used OCs for less than one year or never. The increased risk for triple-negative breast cancer was 4.2 among women 40 years of age or younger who used OCs for more than one year, while there was no increased risk for women between the ages of 41 and 45. Also, as duration of OC use increased, triple-negative breast cancer risk increased.

Breast cancer classification is used to assess the tumor to decide on treatment and prognosis. Classification can be performed using molecular, immunohistochemical, and clinical characteristics. One of the important classification types is receptor status, because it identifies those cancers that have specific targeted treatments available. Breast cancer tumors have traditionally been classed using immunohistochemistry as one of four types: