MCDB 131 Medium contains many components not found in traditional basal media, such as trace elements, putrescine, adenine, thymidine, and higher levels of some amino acids and vitamins. These additions allow the medium to be supplemented with very low levels of serum or defined components. MCDB 131 Medium contains no proteins or growth factors, and is often supplemented with EGF, hydrocortisone, glutamine, and low levels of Fetal Bovine Serum (FBS). The FBS concentration must be optimized for each cell type. MCDB 131 Medium uses a sodium bicarbonate buffer system (1.176 g/L), and therefore requires a 5–10% CO2 environment to maintain physiological pH.