<Li> GTP is added to the terminal bisphosphate by mRNA guanylyltransferase, losing a pyrophosphate from the GTP substrate in the process . This results in the 5 ′--5 ′ triphosphate linkage, producing 5' (Gp) (ppN) (pN); </Li> <Li> The 7 - nitrogen of guanine is methylated by mRNA (guanine - N7 -) - methyltransferase, with S - adenosyl - L - methionine being demethylated to produce S - adenosyl - L - homocysteine, resulting in 5' (m7Gp) (ppN) (pN) (cap - 0); </Li> <Li> Cap - adjacent modifications can occur, normally to the first and second nucleotides, producing up to 5' (m7Gp) (ppN *) (pN *) (pN) (cap - 1 and cap - 2); </Li> <Li> If the nearest cap - adjacent nucleotide is 2' - O - ribose methyl - adenosine (i.e. 5' (m7Gp) (ppAm) (pN)), it can be further methylated at the N6 methyl position to form N - methyladenosine, resulting in 5' (m7Gp) (ppm6Am) (pN). </Li>

The 5'-terminal cap structure of eukaryotic mrnas is a(n)