Reactive oxygen species (ROS) include superoxide anion (O₂⁻), hydrogen peroxide (H₂O₂), hydroxyl radicals (·OH), and others. They are primarily generated by NADPH oxidase on mitochondria, endoplasmic reticulum, and cell membranes. ROS participate in signaling, cell proliferation, and differentiation at low concentrations. Elevated ROS levels lead to oxidative stress, damaging proteins, lipids, DNA, and are implicated in inflammation, aging, cancer, among other conditions. Cellular defense mechanisms include antioxidant enzymes (such as SOD, CAT) and non-enzymatic antioxidants. Gene-specific expression and mutations influence ROS production and clearance, correlating with various diseases.