Why are human G-protein-coupled receptors predominantly intronless?

The proportion of intronless human genes is generally thought to be low—at most 5%. One of the few families of intronless genes is the histones: comparatively small proteins (average 130 amino acids, range ∼100–220 amino acids) that are abundantly expressed and have a high degree of protein sequence conservation. A striking group of larger human genes (average 380 amino acids, range ∼300–880 amino acids), which has many members with intronless protein coding regions, is the G-protein-coupled receptor family (GPCRs)1. An extensive survey of human and other mammalian GPCR sequences from GenBank, excluding genes with incompletely specified coding regions or which were more than 90% similar to other GPCRs at the protein level, revealed that in excess of 90% are intronless in their open reading frame (ORF).