An exome is the collection of exons, the sequences within the primary mRNA transcripts of an organism that are eventually used for making proteins. The rest, non-coding sequences, called introns, are removed by RNA splicing. (Note that there is no 'introme', at least not yet.)
In eukaryotes, exons make up a small proportion of the genome, often around 1% of the total. In prokaryotes, exons constitute its majority, introns being relatively rare. In eukaryotes, therefore, sequencing the exome is more convenient – and cheaper – than sequencing the whole genome. Obviously, this is of less interest with the prokaryotes. Consequently, the term exome is used mainly in connection of analyzing eukaryotic genomes. This has important implications, for example, in sequencing protein coding regions of individual humans to determine if they carry mutations that might be associated with a particular disease. However, important information not contained in the exome will be missed using this method in comparison to whole genome sequencing.
Exome sequencing, a child of next-generation sequencing, is usually done in two steps: capturing the exons and sequencing them. The first step is usually carried out by using custom-made single-stranded oligonucleotides (probes) to which genomic fragments will hybridize. After these fragments are sequenced, they are subjected to one of several kinds of statistical analyses. The quality of exome sequence data can be improved by various methods.