Fig. 11

Frequency of substitutions affecting isomiR sequences. a Percentages of nucleotide substitutions were computed based on unique sequence counts. On average, G nucleotides were more frequently modified (30% of isomiR counts) than C/U (27%) and A (17%), being mainly replaced by A or U. Altogether, G > A, U > C and C > U substitutions were the most frequent polymorphisms, each accounting for ~ 15% of all polymorphisms. b The distribution of variations along the miRNA sequence clearly shows that substitutions are more likely to occur within the 3p region (61% of isomiRs, ~ 7% substitution per nucleotide) than the first 5p nucleotide (4%) or the seed region (16% of isomiRs, ~ 2% substitution per nucleotide). Substitutions at 5p were prone to be G or U nucleotides (33% and 28%, respectively), whereas substitutions to A or U nucleotides were observed more frequently at 3p (34% and 33%, respectively)