Eukaryotic mRNA (blue) has a polyA tail. A DNA copy (cDNA) can be made with reverse transcriptase using oligo dT as a primer (red). After the RNA is destroyed by NaOH treatment, double-stranded DNA can be made with DNA polymerase using random hexamer primers (green). The total population of cDNAs can be cloned as a cDNA library, or a particular cDNA can be amplified by PCR using gene-specific primers.