Estrogen receptors are proteins that bind to estrogen hormones. In some breast cancer cells, these receptors promote cancer growth. Testing for ERs helps determine whether a breast cancer is likely to respond to hormone therapy.

Test Purpose
- **Diagnosis**: To determine if breast cancer cells have estrogen receptors, which indicates whether the cancer may grow in response to estrogen.
- **Treatment Planning**: To decide if hormone therapies like tamoxifen or aromatase inhibitors will be effective.

The presence or absence of estrogen receptors helps in assessing the risk of cancer progression and planning the most appropriate treatment strategy. ER-positive cancers often have a better prognosis and are generally treated with hormone therapies to reduce the risk of recurrence.

- The results are often reported as either positive or negative:
  - **ER-Positive**: The cancer cells have estrogen receptors. This generally suggests that hormone therapy could be effective.
  - **ER-Negative**: The cancer cells do not have estrogen receptors. Hormone therapy is less likely to be effective.

- **ER-Positive (e.g., ≥1% positive cells)**: Indicates that the cancer may respond to hormone therapy. The higher the percentage of ER-positive cells, the more likely hormone therapy will be beneficial.
- **ER-Negative (e.g., <1% positive cells)**: Hormone therapy is less likely to be effective. Other treatment options may be considered.

- **Tissue Sample**: Usually obtained from a biopsy or surgery. The tissue sample is then analyzed in a laboratory.