A method is described for the detection of certain nucleotide modifications adjacent to the 5' 7-methyl guanosine cap of mRNAs from individual genes. The method quantitatively measures the relative abundance of 2'-O-methyl and N(6),2'-O-dimethyladenosine, two of the most common modifications. In order to identify and quantitatify the amounts of N(6),2'-O-dimethyladenosine, a novel method for the synthesis of modified adenosine phosphoramidites was developed. This method is a one step synthesis and the product can directly be used for the production of N(6),2'-O-dimethyladenosine containing RNA oligonucleotides. The nature of the cap-adjacent nucleotides were shown to be characteristic for mRNAs from individual genes transcribed in liver and testis.