NOX4 and cancer: Previous studies have demonstrated upregulation of the expression of the NADPH oxidases NOX1, NOX4, and NOX5 in breast cancer, suggesting mitochondrial H2O2 production by NADPH oxidase NOX4, which in turn protects cancer cells against etoposide cytotoxin-induced cell death, increases invasion, anchorage-independent growth, and EMT (28, 113).