The inhibition of cancer cell growth exerted by the five considered compounds (1–5) has been evaluated in vitro against two models of breast cancer, namely the estrogen receptor positive (ER+) MCF-7 and the triple negative MDA-MB-231 human breast cancer cells (lacking ER, PR, and HER-2/Neu amplification), together with the normal counterpart, namely the non-malignant breast epithelial cells MCF-10A. The gene discussed is ESR1; the disease is breast cancer.