At some point in cell culture, cells must be preserved and frozen. Cryopreservation minimizes changes in cellular genetics and provides a valuable source of backup cells for future use. In ensuring the survival of cells in the freezing and thawing processes, specialized media may be required especially for more sensitive mammalian cells. To reduce freezing damage, such media typically includes a cryoprotectant; glycerol or dimethyl sulfoxide (DMSO) are often used. Now a variety commercial freezing reagents are available, with features such as being chemically defined, serum-free, or tailored for specific primary cell types.