Frequency of different classes of RNA species present in sRNA libraries. The sequences obtained from the sRNA libraries were subjected to a series of sequence similarity searches using specific databases (rRNAs, tRNA, sn/snoRNAs, miRNAs, other non-coding RNAs) and the pipeline described in Figure 1. The sequences that did not match with any known sequence were matched against databases of intergenic and intronic regions of the human genome. The pie-charts represent an overview of small RNA gene expression (shown in percentage) in normal PBMC and two cancer cell lines K562 and HL60. Small RNAs belonging to the miRNA family constitute the majority as in normal PBMC (61%) and HL60 (77%) samples. However, in K562 miRNAs constitute only 18% of the sRNA population.