Figure shows chemical structures of commonly seen base modifications and rare bases. Cytidines or uridines can be methylated at the N-5 position to be 5-methylcytidines or 5-methyluridines. Cytidines can have the oxygen replaced with a sulfur at the N-2 position to be 2-thiocytidines. When uridines have the oxygen replaced with a sulfur at either the N-2 or the N-4 position, they become either 2-thiouridines or 4-thiouridines. Uridines can also be reduced on the base ring and become 5,6dihydrouridines. Another commonly seen base modification is methylation at the N-7 position of the guanosine, which often occurs for the mRNA cap modifications. The guanosines can also be modified at the N-7 position by converting the nitrogen to a carbon and become 7-deazaguanosine. Similar modifications can happen to adenosines resulting in 7-methyladenosines and 7-deazaadenosines. Pseudouridines and inosines are also regularly seen as rare bases in modified RNA sequences. Additional modified or rare bases are shown in Supplement Figure . According to the data in the CAS Content Collection , the use of RNA modifications has been increasing since the beginning of 1995 (Supplemental Figure ). Base modifications that interfere with the formation of hydrogen bonds can thermally destabilize duplex formation with the target and thus improve target specificity by limiting off-target binding. In addition, modifications improve the performance of therapeutic RNA. Replacing uridine with the modified base 1-methylpseudouridine (Figure ) in therapeutic mRNAs, such as the COVID-19 vaccines (Pfizer's Comirnaty and Moderna's Spikevac ), improves translation and lowers cytotoxic side effects and immune responses to the mRNA. 257 Both Pfizer's Comirnaty and Moderna's Spikevac mRNA vaccines also use a 7-methylguanosine cap linked by a 5' triphosphate to the 5' end of the mRNA, replicating the normal mRNA caps that prevent degradation of the 5' end of the mRNA. 258