Cyanine5 hydrazide is a reactive dye for the labeling of aldehydes and ketones, an analog of Cy5 hydrazide. This dye reacts smoothly and nearly quantitatively with various carbonyl groups encountered in biomolecules. Examples are proteins subjected to oxidative stress, glycosylated proteins pre-activated by periodate oxidation (including antobodies), and oligonucleotides with aldehyde moieties. Cy 5 hydrazide is a fluorescent dye for aldehydes and ketones, suitable for protein labeling