Why do they treat some woman with breast cancer with herceptin (trastuzumab)?

Some have her-2-neu: Expressed; some do not. It only works on those who express sufficient her-2-neu copies by fish (fluoroescent in-sit hybrization), a test done on the cancer tissue at the pathology lab.

Herceptin (trastuzumab) breast ca: About 1/4 cases of breast cancer express a protein on the surface of the cancer cells called her2neu which is a member of the epidermal growth factor family of receptors important for growth signaling into the cell nucleus. Herceptin (trastuzumab) is an antibody developed to target her2neu and antagonize this growth signaling and kill the cancer cell. Chemo is synergistic with Herceptin (trastuzumab) when given together.