As an additional pharmacodynamic analysis in MCF7 xenografts, we next monitored tumor 18F-fluoroestradiol (FES) uptake by positron emission tomography (PET), which has previously been used as a non-invasive, real-time measure of the activity of ER modulators [reviewed in (Liao et al., 2016)]. The gene discussed is ESR1; the disease is neoplasm.