(A) Model for miRNA biogenesis. The initiation step is mediated by the Drosha complex in the nucleus. The product of this nuclear processing step is an ∼ 70-nucleotide pre-miRNA. After export, the cytoplasmic RNase III dicer participates in the second processing step (dicing) to produce miRNA duplexes. The duplex is separated by helicase and usually one strand is selected as the mature miRNA, whereas the other strand is degraded. (B) Scatter plots of expression levels of miR-21, miR-19b, and miR-92a in CSF samples from 23 PCNSL patients compared with 30 control patients with various neurologic disorders. Increased mean and median relative expression levels of miR-21, miR-19b, and miR-92a were demonstrated in PCNSL patients' CSF compared with controls. Professional illustration by Debra T. Dartez.