Description: All cells require iron for the proper uptake of oxygen from their environment. Transferrin, a protein isolated from serum, causes the uptake and transport of iron from culture medium to the cells. This allows proper oxygen uptake and also stimulates growth-related enzyme activity. Iron-saturated transferrin (holo transferrin) is continually recycled after releasing iron and becomes iron-deficient transferrin (apo-transferrin). Choosing which form to use depends upon the culture conditions. In media such as Dulbeccos Modified Eagles Medium/Hams Nutrient Mixture F12 (DMEM/F12) that contain high levels of iron salt, using apo-transferrin is preferred.