A method is described whereby new cDNA or RNA sequences can be introduced into or substituted for cDNA in any chosen position without specific sequence requirements using the polymerase chain reaction (PCR). The method entails the use of primers which are complementary to the 3' and 5' ends of the desired sequence to be inserted as well as to the 3' and 5' ends of the chosen site of insertion in the acceptor molecule. The desired sequence is amplified by PCR such that single stranded fragments are produced. The single stranded fragment of the desired sequence is then annealed to a single stranded acceptor molecule at the site of insertion and extended to produce a double stranded molecule. The double stranded molecule is then separated into two strands which are identical except that one of the strands contains the desired sequence inserted at the chosen site. A second double stranded molecule is then generated.