If you have samples you use frequently, such as reference samples or positive samples, we recommend making aliquots and storing these at -20°C. One aliquot at a time can then be stored at 4°C.
DNA extracted for conventional MLPA or digitalMLPA analysis should ideally be stored in TE0.1 (10 mM Tris-HCl pH 8.0 + 0.1 mM EDTA), as this increases stability as opposed to storage in water. TE0.1 is also more suitable for (digital)MLPA reactions; see this article for more information.
Long-term storage at 4°C may degrade sample quality. Evaporation can increase the concentration of impurities, such as EDTA, which can inhibit downstream reactions. The growth of microorganisms can also degrade sample quality and introduce metabolites that can inhibit reactions.