Oligo d(T)12-18 is the classic primer mix used to prime synthesis of the first strand cDNA by reverse transcriptase using poly A+ mRNA as a template. Oligo dT of various sizes are synthesized individually and gel purified. Oligo d(T)12-18 is a mixture of individually synthesized and purified primers of varying sizes. These are mixed in equimolar ratio. Oligo purity is greater than 98% as determined by denaturing polyacrylamide gel electrophoresis.