Distribution of positions of observed substitutions in sRNA sequences. Sequences were obtained with the MPSS method (A), with 454 technology by Lu et al.  (B), and our own sequence set (C). For each graph, the left side shows the number of substitutions associated with initial nucleotides (where "1" is the first nucleotide) and right side shows the number of substitutions associated with terminal nucleotides (where "-1" is the last nucleotide). For the entire data set, substitutions were especially abundant in the initial/terminal three nucleotides.