For example, a recent study found that using a transition‐metal boride nanozyme (molybdenum boride, MoB2) to induce reductive stress had unintended pro‐tumoral effects on the immune microenvironment (Figure 9C).[128] In that work, MoB2 nanozymes potently scavenged ROS in the tumor, but this led to an immunosuppressive milieu: macrophages in the treated tumors polarized toward the M2 phenotype (immunosuppressive, pro‐tumor) and away from the tumor‐fighting M1 phenotype. The gene discussed is MOB2; the disease is neoplasm.