In eukaryotes, the majority of mRNAs have an m(7)G cap, which is added cotranscriptionally and plays a critical role in many aspects of mRNA metabolism. The effect of the cap on translation is mediated by the initiation factor eIF-4F, whereas the effect on pre-mRNA splicing involves a nuclear complex(CBC). CBC consists of two cap binding proteins CBP20 and CBP80, which mediate the stimulatory functions of the cap in pre-mRNA splicing, 3' end formation and U snRNA export. The genes CBC1 and CBC2 encode CBP80 and CBP20, respectively. CBP80 comprises three domains, each containing a MIF4G domain. CBP20 has an RNAP fold and associates with the second and third domains of CBP80. CBP also plays a role in nonsense-mediated decay(NMD), which eliminates mRNAs, which prematurely terminate translation.CBP80-bound mRNA undergoes a “pioneer” round of translation beforeCBP80-CBP20 are replaced by eIF4E, and Upf2 and Upf3 proteins.