Sequence data for RNAs and RNA modifications are annotated and collected with CAS Registry Sequence Guidelines 264 from published documents and stored in the CAS Content Collection 83 . To better understand the chemical modification trends on RNA molecules, we extracted ~170,000 modified RNA sequences from the CAS Content Collection. Figure shows the number of the modified RNA sequences distribution of these modified sequences along the sequence length. The predominance of modified nucleotide RNAs for lengths of 18-27 bases reflects the fact that this sequence length is commonly used in siRNAs and ASOs; processed, naturally occurring double-stranded siRNAs are typically 21 or 23 bp long. The double-stranded nature of siRNAs accounts for the large number of modifications for nucleotides with a length of 42 and 44; two 21-nucleotide RNAs produce 42 nucleotides of RNA, while a 21-nucleotide and a 23-nucleotide RNA produce 44 nucleotides. The percentage of modified RNA sequences in the total RNA sequences was also shown along the sequence length, suggesting that sequences less than 100 base pairs are more frequently being modified than the longer sequences. Chemical modifications were further analyzed using this data set with specific types of modifications and sequence lengths. A heatmap (Figure ) was constructed based on the relative frequencies of specific types of modification in the total modification event in that sequence length. The heat map of modification type versus sequence length shows a sharp change in the types of modifications in sequences <100 nucleotides vs. >100 nucleotides. Sequences >100 nucleotides are modified much less than shorter sequences, which reflects the use of chemical synthesis methods like solid-support synthesis to produce small RNAs and the use of in vitro transcription to produce larger RNAs. Longer sequences are distinct with more triphosphates and 7-methylguanosines, suggesting that they are mRNAs with 5' end caps consisting of 7-methylguanosine linked to the 5' end of the mRNA with a triphosphate group. Since therapeutic mRNAs are translated by the ribosomes to produce an active protein, excessive modification might provide steric hindrance that inhibits translation. The dominant nucleobase modifications for RNAs <100 nucleotides long are 5-methyluridine (m 5 U) and 5-methylcytosine (m 5 C). The rare base pseudouridine appeared frequently in the database, followed by the rare bases dihydrouracil and inosine. Bases in which the N-7 position is converted to a carbon (7-deazaguanine and 7-deazaadenine) appeared with some frequency as did the replacement by sulfur of the oxygen double-bonded to the N-4 or N-2 position of uracil or the N-2 position of cytosine. Less common modifications included benzoyl groups attached to N-3 in cytosine and the nitrogens in adenosine.