Breast Cancer: Is the ER (estrogen receptor) something genetic or is it variable based on medication (ie. chemotherapy)?

Receptors are proteins in or on cells that can attach to certain substances in the blood. Normal breast cells and some breast cancer cells have receptors that attach to the hormones estrogen and progesterone, and depend on these hormones to grow.
The breast tissue estrogen receptor is both genetic and variable in the presence of chemotherapy and other medications.
Breast cancer cells may have one, both, or none of these receptors.
ER-positive: Breast cancers that have estrogen receptors are called ER-positive (or ER+) cancers.
PR-positive: Breast cancers with progesterone receptors are called PR-positive (or PR+) cancers.
Keeping these receptors from attaching to the hormones can help keep the cancer from growing and spreading. There are drugs that can be used to do this.
Knowing the hormone receptor status of your cancer helps doctors decide how to treat it. If your cancer has one or both of these hormone receptors, hormone therapy drugs can be used to either lower estrogen levels or stop estrogen from acting on breast cancer cells. This kind of treatment is helpful for hormone receptor-positive breast cancers, but it doesn’t work on tumors that are hormone receptor-negative (both ER- and PR-negative).