TYRP1 and breast cancer: Next, we evaluated the cytotoxicity and CO‐releasing capability of ET‐CORM in two CatB‐expressing breast cancer cell lines, SKBR3 and MCF7, which are reported to have comparable CatB expression levels.[55, 56] In both models, ET‐CORM exhibited low cytotoxicity over 48 h (IC50 > 100 μM, Figure 1a), similar to previously reported enzyme‐triggered CORMs bearing the same iron core.