Unique molecular identifiers (UMIs) are short indices used to tag each molecule within a sequencing library uniquely. UMIs consist of random sequence compositions, which assure the unique fragment-UMI combination in the library. These molecular barcodes are added to a sequencing library before PCR. Therefore, UMI enable the accurate quantification of the original nucleic acids with bioinformatics software by removing duplicate reads and PCR errors. Distinguishing PCR duplicates from real biological duplicates, results in improved data quality and increased variant detection sensitivity.
Do not mix up UMIs and UDIs. UDIs are unique dual indexes. They allow the assignments of reads with the same barcodes to a specific sample after pooling (see “What is multiplexing and demultiplexing”) and have to be used in each library preparation. When UMIs are available for special library preparation, they can be used additionally to the UDIs. Combining both UMIs and UDIs can improve data analysis accuracy.