4 Hydroxynonenal (also known as 4-hydroxynonenal, 4-HNE) is a highly reactive aldehyde generated by the exposure of polyunsaturated fatty acids to peroxides and reactive oxygen species (ROS). It non-enzymatically forms stable protein adducts with histidine, lysine, and cysteine side chains that have been used as biomarkers for oxidative damage in cells. Conditions where 4-HNE immunoreactivity has been observed include include inflammation, neurodegenerative diseases, and ischemic damage to the heart and brain.