It is a method for increasing specificity of PCR reactions. Touchdown PCR uses a cycling program where the annealing temperature is gradually reduced (e.g. 1-2°C /every second cycle). The initial annealing temperature should be several degrees above the estimated Tm of the primers. The annealing temperature is then gradually decreased until it reaches the calculated annealing temperature of the primers or some degrees below. Amplification is then continued using this annealing temperature.