While the indene-based T6 molecule resulted in a worse IC50 value than its unmodified or ferrocene-linked counterpart in MDA-MB-231 and PANC1 cells, it led to a tenfold improvement in antitumor effect on the classical ER-positive MCF7 breast cancer cell line at 72 h (IC50: 4.9 μM—see Table 1). The gene discussed is ESR1; the disease is breast cancer.