The colony inhibition assay (10 μM treatment over 7 days) using the MCF‐7 breast cancer cell line was particularly instructive as a measure of Hsp90's on‐target effects on clonogenicity, with the most notable reduction in colony formation (< 50% remaining) for test compounds 5d (R1 = 2‐F, 4‐OMe, R2 = 3,4‐OCH2O‐; 49.75%) and 5i (R1 = 4‐Cl, R2 = 4‐OMe; 8.39%). Here, HSP90AB1 is linked to breast cancer.