The estrogen receptor level measured in a primary tumor or in metastases. There are essentially two distinct forms of breast cancer, one that tends to have an early onset and is characterized by an ER- status, and one that tends to have a late onset and is characterized by an ER+ status. However, these two variants tend to overlap with respect to age, so it is possible for a young woman to have a ""late-onset"" form of breast cancer and for an older woman to have an ""early-onset"" variant of the disease. [from
NCI]