5x PCR buffer is a 5x reaction buffer suitable for most PCR applications. Standard 5x PCR buffer contains 10 mM MgSO4, which corresponds to 2 mM MgSO4 in the reaction mixture. If it is necessary to select the concentration of Mg2+ ions in the reaction mixture, it is recommended to use a PCR buffer that does not contain magnesium salts (together with a 100 mM magnesium sulfate solution).