NPs can also enter cancer cells by receptor-mediated endocytosis in the case of pro-oxidant activity as illustrated in Figure 3C. The acidic intracellular pH of cancerous cells stimulates the NPs to mimic SOD-like activity, which reduces superoxide into H2O2 but inhibits CAT-mimetic activity, causing a considerable amount of H2O2 to be generated in the cancer cell. The gene discussed is SOD1; the disease is cancer.