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Why is the Tm on my spec sheet different from the calculated Tm I obtained when I designed my oligos?

Oligonucleotides designed using our online SciTools® programs are ordered for a wide variety of applications, and not solely for PCR. The Tm shown on the spec sheets is calculated without taking into consider­ation Mg2+ and dNTP concentrations, which influence Tm. To determine the correct Tm for your oligo under your specific reaction conditions, enter the sequence into the on­line OligoAnalyzer® tool, making sure to change the defaults to your reactions conditions.