Cy5.5 is a near-infrared fluorescent dye with an Ex/Em of 680/710 nm, commonly used for labeling proteins, nucleic acids, and cells in various imaging and detection applications. It is available as a 5 mg/mL stock solution in PBS (5.45 mM) and can be used for protein labeling, e.g., 8 µL for 500 µL of 2 mg/mL IgG (MW=150,000).