The “exome” is the region of the DNA that specifically encodes protein.

FTA:

Unlike earlier diagnostics that study one gene at a time, this test rapidly sifts through all of the 37-million base pairs in a person’s 20,000 genes to tease out the single DNA change causing a rare genetic disorder. It focuses on the exome, the protein-coding portions of genes that account for only 1 percent of DNA but nearly 85 percent of the glitches known to cause human diseases.

Also relevant… the price. FTA:

With preauthorization, many insurance providers cover the cost to sequence a child and both parents. If not, the out-of-pocket fee runs $6,650.