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 the name triple-negative). Triple-negative is sometimes used as a surrogate term for basal-like. Triple-negative breast cancer comprises 15¨C20% 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¨C5 years, but drops sharply and substantially below that of hormone-positive breast cancers afterwards.

Reference
Wiki: Triple-negative breast cancer