RNA-seq (RNA Sequencing) is a revolutionary technology that uses next-generation sequencing to identify the presence and quantity of RNA in a biological sample at a given moment. It is based on the similar principle as DNA sequencing with one difference. RNA-seq is carried out to obtain the order of nucleotide sequences of a target RNA molecule instead of a DNA molecule. 

During the RNA-seq process, all the RNA molecules are first sequenced and then mapped back to the genome. This gives researchers detailed insight into gene expression, fusions, alternative gene-spliced transcripts, and mutations.