Drug conjugate 6a with a shorter linker than in 6b (five CH2-groups vs. six CH2-groups) is equipotent to 6b at ERα but is surprisingly 100-fold more potent as EGFR inhibitor (EC50 = 2.5 nM vs. 260 nM) making 6a a more promising anti-cancer agent. This evidence concerns the gene ESR1 and cancer.