The concentration of calcium found in classic media and their derivatives varies from 0 in BGJb Medium Fitton-Jackson Modification, developed to culture cartilagenous embryonic bone, to 2 mM in MCDB media 201, developed to culture Chick Embryo Fibroblasts, an attached cell line. As a general rule, the highest concentrations of calcium are found in serum supplemented, such as FBS, classic media developed for attached cell types. Sera generally protects cells from oxidative damage. Calcium levels are generally lower in serum-free media and media developed for primary and clonal cell culture. This may be due to the fact that primary cells and cells grown at clonal densities are susceptible to the toxic influx of calcium through cell membranes damaged by oxidative processes, prevalent in serum-free culture media, and enzymatic and mechanical processing. Since calcium affects cell signaling and differentiation as well as cell attachment, media developed for specific cell types may require substantially lowered calcium levels.