A 7-methylguanosine triphosphate group is added to the 5' end of the pre-snRNA during transcription elongation (Mattaj 1986). The capping enzyme and cap methyltransferase involved in mRNA capping may also be responsible for this reaction. In the case of mRNA capping, the capping enzyme is targeted to the pre-mRNA by interaction with the phosphorylated C-terminal domain (CTD) of RNA polymerase II (McCracken et al. 1997). During elongation, the phosphorylation pattern of the CTD also changes: serine-5 is dephosphorylated by RPAP2 (Egloff et al. 2012) interacting with RPRD1A and RPRD1B (Ni et al. 2011, Ni et al. 2014) and serine-2 is phosphoryated by P-TEFb. Serine-7 is also phosphorylated, possibly, however the responsible kinase is not certain. The order of the capping and phosphorylation events is unknown.