cDNA stands for complementary DNA, which is synthetic DNA that has been transcribed from a specific mRNA through a reaction using the enzyme reverse transcriptase. While DNA is composed of both coding and non-coding sequences, cDNA contains only coding sequences. Scientists often synthesize and use cDNA as a tool in gene cloning and other research experiments. cDNA is often used to express a specific protein in a cell that does not normally express that protein, or to sequence or quantify mRNA molecules using DNA-based methods. cDNA that codes for a specific protein can be transferred to a recipient cell for expression, often bacterial or yeast expression systems. cDNA is also generated to analyze transcriptomic profiles in bulk tissue, single cells, or single nuclei in assays such as microarrays, qPCR, and RNA-seq. cDNA is also produced naturally by retroviruses and then integrated into the hosts genome, where it creates a provirus. The term cDNA is also used, typically in a bioinformatics context, to refer to an mRNA transcripts sequence, expressed as DNA bases rather than RNA bases.