Transferrin is a glycoprotein that binds iron for delivery to vertebrate cells through receptor-mediated endocytosis. After binding to its receptor on the cell surface, transferrin is rapidly internalized by invagination of clathrin-coated pits with formation of endocytic vesicles, the acidic environment favors dissociation of iron from the transferrin-receptor complex. Following the release of iron, the apotransferrin is recycled to the plasma membrane where it is released from its receptor to scavenge more iron. Fluorescent conjugates of human transferrin can be used as tracers for investigation of endocytosis by microscopy. CF® dyes are Biotium's line of next-generation fluorescent dyes with advantages in brightness, photostability, and conjugate specificity compared to other fluorescent dyes. Red fluorescent CF®568 dye has excitation/emission at 562/583 nm.