An oligodeoxynucleotide primer hybridized to mRNA is extended by an RNA-dependent DNA polymerase to create a cDNA copy that can be amplified by PCR.  Depending on the purpose of the experiment, the primer for first-strand cDNA synthesis can be specifically designed to hybridize to a particular target gene, or a general primer such as oligo(dT) can be used to prime cDNA synthesis from essentially all mammalian mRNAs

Protocol: Amplification of cDNA Generated by Reverse Transcription of mRNA Protocol