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By Mayo Clinic Staff

A HER2-negative breast cancer diagnosis can feel confusing and overwhelming at first. People with this diagnosis often have many questions about this cancer and how it is treated.

Breast cancer begins as a growth of cells in the breast tissue. Breast cancer starts when something changes the DNA inside cells in the breast tissue. A cell's DNA holds the instructions that tell a cell what to do. In healthy cells, the DNA gives instructions to grow and multiply at a set rate. The instructions tell the cells to die at a set time. In cancer cells, the DNA changes give different instructions. The changes tell the cancer cells to make many more cells quickly. The cancer cells might form a mass called a tumor. The tumor can grow to invade and destroy healthy body tissue.

Breast cancers, including HER2-negative breast cancer, are often grouped into types based on the results of tests on the cancer cells. Lab tests can show whether the cancer cells use hormones such as estrogen or progesterone or a protein called human epidermal growth factor receptor 2 (HER2) to help them grow.

Whether the cancer uses one, all or none of these determines the cancer type. The types, including HER2 results, will give your healthcare team important clues on how to treat the cancer.