Sample indexes (or indices) enable multiple samples to be sequenced together (i.e., multiplexed) on the same instrument flow cell or chip. Each sample index, typically 8–10 bases, is specific to a given sample library and is used for de-multiplexing during data analysis to assign individual sequence reads to the correct sample. Predesigned indexes from IDT are balanced to prevent GC bias and color-balanced for 2- and 4-color Illumina instruments. Adapters and primers may contain single or dual sample indexes depending on the number of libraries combined and the level of specification desired. Illumina recommends using unique dual indexes (UDIs) as a method to mitigate errors introduced by index-hopping. UDIs are particularly important when using instruments with patterned flow cells, such as the NovaSeq® system from Illumina.
IDT provides several indexing options with its Custom NGS Adapters. These include both IDT and Illumina 8- and 10-base index series, as well as the ability to upload your own index files.
Unique molecular identifiers (UMIs) provide reliable error correction. UMIs are short sequences that incorporate a unique barcode onto each molecule within a given sample library.
- UMIs are used to identify PCR duplicates. PCR duplicates created during library amplification steps will share the same insert and UMI tag.
- UMIs have also been shown to reduce the rate of false-positive variant calls and increase variant detection. By incorporating individual barcodes on each original DNA fragment, variant alleles present in the original sample (true variants) can be distinguished from errors introduced during library preparation, target enrichment, or sequencing. Any identified errors can be removed by bioinformatics methods before final data analysis.
Adapters that contain UMIs, such as the xGen UDI-UMI Adapters, are available with a UDI design for identification of low-frequency variants. IDT Custom NGS Adapters can also be configured with UMIs.