Tissue-specific cDNA is generated from the reverse transcription of total RNA extracted from tissues or organisms. In choosing such cDNA, one aspect to pay attention is the type of primers used to synthesize the reads. Oligo-dT primers are a chain of 12-18 deoxythymidine bases that pair with the polyadenine (polyA) tails of eukaryotic mRNAs. These primers are ideal for constructing cDNA libraries as they are selective for the protein-coding subpopulation of RNA. However, these primers will miss RNA lacking polyA tails, such as prokaryotic RNA, partially degraded RNA from fixed samples, and microRNA. On the other hand, six-nucleotide long random primers, also known as random hexamers, can be used to generate reads across all expressed RNA, including structural, small, and other non-coding RNA. They are not suitable, however, for the full length transcription of longer RNA sequences.