It is assumed that the alteration in hormone levels associated with these polymorphisms is responsible for the observed differences in the risk for breast cancer (Kristensen et al., 2000; Talbott et al., 2008; Sergentanis and Economopoulos, 2010), bone loss and fractures (Masi et al., 2001; Gennari et al., 2004; Somner et al., 2004), and response to aromatase inhibitors (AI) in certain gene variants (Colomer et al., 2008; Garcia-Casado et al., 2010). The gene discussed is CYP19A1; the disease is breast cancer.