In zebrafish embryos, 21 (at 12.5 μM) did not lead to noticeable signs of toxicity over 96 h, which together with the high antiproliferative and anti-aromatase activities prompted the authors to consider this complex a suitable candidate for further investigation as a potential anticancer agent against ER(+) breast cancer [79]. This evidence concerns the gene ESR1 and breast carcinoma.