The synthesis of complementary DNA from mRNA templates by the action of reverse transcriptase (RT’ase) is a fundamental technique in molecular cloning. The prime consideration is that a large amount of long cDNA copies should be made. The quality and length of the cDNA product is largely dependent on the quality of the mRNA used as the starting material, and great care must be taken to avoid degradation. The choice of primers and RT’ase used during the synthesis also influences the final product.