9. To determine a number of specific platelet functions before and after 3 months of letrozole and exemestane treatment. This is a sub-study that will be performed only at the Indiana University site. Platelet function will be measured by ex vivo platelet aggregation tests. Production of regulators of platelet function, including thromboxane A2 (TXA2), proscyclin (PGI2) and serotonin will also be assessed. These data will allow us to test the hypothesis that genetic polymorphisms in candidate genes in estrogen-regulated pathways alter the effect of letrozole and exemestane treatment on platelet activity, which may be relevant to their effects on cardiac risks.