Thermal Cyclers are used to amplify segments of DNA via the polymerase chain reaction (PCR) process. The instrument has a thermal block with holes where tubes holding the PCR reaction mixtures can be inserted. The cycler then raises and lowers the temperature of the block in discrete, per-programmed steps. This instrument helps heating and cooling a DNA sample in order to perform the PCR. Today, there are numerous Thermal Cyclers available from a variety of vendors. The instruments differ in the number of samples that can be handled, the size of reaction tube used, the speed of temperature change, and the cost.