Since their introduction as improved agents for in vitro and in vivo cell tracking,34,56,79 the lipophilic membrane intercalating fluorochromes known as PKH dyes have been used to study a wide variety of cell types and biological processes. Originally developed by Horan and colleagues at Zynaxis Cell Science as part of a larger class of multifunctional drug delivery molecules known as Zyn-Linkers, this family of patented fluorescent dyes and cell labeling reagents became available in kit form for research use in 1989 and was purchased by Phanos Technologies in 1995. PKH dyes are therefore found in the literature under a variety of names, including Cell Linker dyes, Zyn-linkers, Zyn-linker® dyes, cell tracking dyes, and PKH dyes. A number of suppliers are also found in the literature, including Zynaxis Cell Science (which no longer sells these reagents), Sigma and Dainippon Pharmaceuticals Laboratory Products Division (both of which continue to distribute PKH kits for Phanos).