As we have shown MDM2 inhibitors to be effective in inhibiting cell proliferation of ESR1-mutant breast cancer cells, and hampering tumor growth of ER-positive breast cancer PDXs resistant to fulvestrant, we believe that MDM2 inhibition might represent an effective alternative therapeutic option for breast cancers refractory to anti-estrogen therapy, often observed among GATA3-mutant breast cancers. The gene discussed is ESR1; the disease is breast cancer.