What is HER2 and how is it tested?
HER2 is a protein that is found in about a quarter to a third of breast cancers. It is a signal that the cell has a higher tendency to divide because it actually is of a family of proteins that are receptors. Not for estrogen, like estrogen receptors, but for other molecules that signal growth. When HER2 is found in a cancer, it means that the cell is responding to those signals because, in general, that whole family of molecules is activated. This is important for several reasons. One, it tells us how rapidly growing the tissue is, which helps us plan therapy. But also, because we actually have a drug that can attack HER2 and can inhibit the growth of these cells; and that drug is called trastuzumab (or, Herceptin), is given intravenously, and has a profound effect at inhibiting the growth and often killing cells that have too much HER2 in them.
