CF® Dye Maleimides are thiol-reactive fluorescent dyes. Maleimides are commonly used to label proteins on free thiol groups. CF® dyes are Biotium's line of next-generation fluorescent dyes with advantages in brightness, photostability, and conjugate specificity compared to other fluorescent dyes. Far-red fluorescent CF®640R dye has excitation/emission at 642/662 nm.