The optimal temperature for cell culture depends on the type of cells being cultured. For mammalian cell lines, the optimal temperature is 37°C for maximum cell growth. The optimal temperature for avian cell lines is 38.5°C. They can also be sustained at 37°C but will experience slower growth. Insect cell lines are grown at an optimal temperature of 27°C. These cells grow slower at lower temperatures and any temperature between 27-30°C. The viability of insect cells decreases above temperatures of 27°C and are unable to return to normal levels once restored at 27°C. Cell lines originating from cold-blooded animals are able to withstand a temperature range between 15-26 °C. In order to store cells properly, a temperature of less than -130 °C is necessary to stabilize cell lines. This is carried out through storage in liquid nitrogen (-196 °C) or in a cryogenic freezer (-150 °C). This process is known as cryopreservation.