Fluorescent biocytin conjugates are cell-impermeant, fixable polar tracers with biotin and an aldehyde-fixable primary amine groups.  Polar tracers are commonly used to investigate cell-cell and cell-liposome fusion as well as membrane permeability and transport through gap junctions or cell uptake during pinocytosis. 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.