This is a research study which aims to improve the way that doctors determine the dose of chemotherapy given to patients. Right now, chemotherapy is determined by a patient's height and weight. However, some patients metabolize chemotherapy faster or slower than the average person because of a different level of drug metabolizing enzyme in the liver. Therefore, some patients are either given too small or too large a dose of chemotherapy because the amount of enzyme is not taken into account. This research study will examine the use of a simple test, call the Erythromycin Breath Test(ERMBT) to determine the amount of enzyme which can metabolize the chemotherapy drug docetaxel (Taxotere). The dose of docetaxel will be tailored to the amount of enzyme which is available to metabolize the drug for each patient. The drug, docetaxel, is combined with another drug, trastuzumab (Herceptin), because at this time this combination appears to be promising in metastatic breast cancer research.