Expression of human circRNAs. (A) Levels of circRNAs in CD34+ hematopoietic progenitor cells. The expression level was estimated for each circRNA (using its circular fraction and the FPKM of the corresponding gene, which included both circular and linear isoforms) and the cumulative distribution of levels is plotted. For comparison, the levels of mRNAs with FPKM ≥0.1 are also plotted. (B) Fractions of mRNA-mapping reads estimated to derive from circRNAs. Reads derived from each circRNA were estimated as the product of the circular fraction, the gene FPKM and the length of the circRNA exonic sequence. The fraction was estimated for each sample, and the distribution of fractions is plotted. (C) Numbers of circRNAs identified in each biological sample. The number of circRNAs was tallied for each sample, and the distribution of values is plotted. (D) Numbers of samples in which ≥10% circular fraction was observed. The number of samples with ≥10% circular fraction was tallied for each circRNA, and the distribution of values is plotted. (E) Cumulative distribution of cell-type-specificity scores of circRNAs compared to mRNAs with similar overall expression levels (linear controls). (F) Unsupervised hierarchical clustering of the circular fractions of 1,299 circRNAs for which the availability of both the donor and the acceptor sites were each supported by ≥5 reads in all 39 samples.