Oxazole yellow, also know as YO-PRO-1, is a green-fluorescent, cell-impermeant, high-affinity carbocyanine monomeric nucleic acid stain.

• Monomeric nucleic acid stain
• Cell-impermeant
• Early marker of apoptosis
• λ_Ex/λ_Em (with DNA) = 491/509 nm
• Supplied at 1 mM in DMSO

It is essentially nonfluorescent in the absence of nucleic acids but exhibits excitation/emission maxima 491/509 nm when bound to nucleic acids. Oxazole yellow is also used to identify apoptotic cells. Early apoptotic cells become permeant to oxazole yellow, but remain impermeant to propidiumiodide (cat. no. 40016), a dead cell stain.