Whole Exome Sequencing (WES) is a DNA sequencing technique that only sequences the parts of DNA with protein instructions (coding regions called exons) – basically, it sequences DNA versions of messenger RNA. This is in contrast to Whole Genome Sequencing (WGS) which sequences everything, including regulatory regions and regions scientists are still trying to figure out what (if anything) they do.
For a good example of one of the first uses of WES to track down the genetic cause of a rare disease, I recommend the book One in a Billion: The Story of Nic Volker and the Dawn of Genomic Medicine by Mark Johnson and Kathleen Gallagher, 2016.
