Use our next-generation sequencing glossary to clarify key terms and important concepts as you plan your sequencing project.
The average number of sequenced bases that align to, or “cover,” known reference bases. For example, a whole genome sequenced at 30× coverage means that, on average, each base in the genome was sequenced 30 times. At higher levels of coverage, base calls can be made with a higher degree of confidence.Learn More
A molecular biology protocol that converts a genomic DNA sample (or cDNA sample) into a sequencing library, which can then be sequenced on an NGS instrument. The first step in library preparation is random fragmentation of the DNA sample, followed by ligation of 5ʹ and 3′ adapters to each DNA fragment. Alternatively, “tagmentation” combines the fragmentation and ligation reactions into a single step and greatly increases the efficiency of the library preparation process.Learn More
A process by which unique short DNA sequences, or “indexes,” are added to each DNA fragment during library preparation. The unique sequences allow many libraries to be pooled together and sequenced simultaneously. Sequencing reads from pooled libraries are identified and sorted computationally before final data analysis. Library multiplexing is a useful technique when working with small genomes or targeting genomic regions of interest. Multiplexing can exponentially increase the number of samples analyzed in a single run, without drastically increasing run cost or run time.Learn More
Having equal proportions of A, C, G, and T nucleotides at each base position across all the DNA fragments in a sequencing library. Color balance is required for effective image analysis on Illumina sequencing systems. Therefore, most Illumina library preparation workflows include a random fragmentation step, which generates the necessary sequence diversity at each base position in the library.Read Bulletin
A metric in NGS that predicts or estimates the probability of an error in base calling. A quality score (Q-score) serves as a compact way to communicate very small error probabilities. A high Q-score implies that a base call is more reliable and less likely to be incorrect.Learn More
Pairs of indexes such that every i5 index and every i7 index are used only one time. With unique dual indexes, you can identify and filter indexed hopped reads, which provides higher confidence in multiplexed samples.Read Bulletin
A widely used sequencing method that targets only the protein-coding region of the genome (the exome).Learn More
Learn about read length, sequencing coverage, and more—everything you need for your first sequencing run.Start Planning
Have questions about our products or how to get started with NGS? Connect with an Illumina representative.
This detailed overview describes major advances in technology, the basics of Illumina sequencing chemistry, and more.
View all our training courses or sign up for upcoming webinars.
Find more sequencing tips and tricks from our Technical Support team.