Although 5d did not significantly inhibit colony formation in the breast cancer cell lines SKBR3 (ER−, PR−, HER2+) and BT474 (ER+, PR+, HER2+), potent effects on surviving colonies were noted in HCC1954 and MDA‐MB‐231 cell lines at 10 μM. This evidence concerns the gene ERBB2 and breast cancer.