In this study, we characterised the anti-tumour effect of p53 activation via MDM2 inhibition using the small molecule inhibitor NVP-CGM097—a dihydroisoquinolinone derivative currently being evaluated in a phase I clinical trial [17, 18]—in endocrine-resistant and endocrine-sensitive in vitro and in vivo models of ER-positive breast cancer. The gene discussed is MDM2; the disease is neoplasm.