Alexa Fluor® 750 C₅-maleimide is spectrally similar to Cy7 dye. As the longest-wavelength Alexa Fluor® dye, the emission is well separated from commonly used far-red fluorophores such as Alexa Fluor® 647 dye or allophycocyanin (APC), facilitating multicolor analysis. Fluorescence of this long-wavelength Alexa Fluor® dye is not visible to the human eye but is readily detected by most imaging systems. As a class of compounds, thiol-reactive dyes are principally used to prepare fluorescent peptides, proteins, and oligonucleotides for probing biological structure, function, and interactions. Because the thiol functional group is not very common in most proteins, thiol-reactive reagents often provide a means of selectively modifying a protein at a defined site. Alexa Fluor® 750 C₅-maleimide is supplied as 1 mg of dry powder and should be stored at <=-20°C, desiccated, and protected from light.