Rapamycin inhibits polysomal partitioning of JUNB mRNA. Ribosomal 17.1% to 51% sucrose gradients were prepared from NPM-ALK–positive ALCL cell line SR-786 treated with 20 nM rapamycin or control (DMSO). Twelve fractions of each gradient were collected from bottom to top after ultracentrifugation, with online monitoring of RNA content via an OD254 UV-flow cell (gray shaded areas). RNA was isolated from each fraction and analyzed by gel electrophoresis (bottom panels), and quantities of JUNB as well as β-actin mRNA were determined using Northern blot analysis on the same blot membranes. As indicated by the photometric OD254 measurement, fractions 3 and 4 represent monosomes; fraction 5, disomes; and fractions 6 and higher, higher polysomes. Mono-polysomal β-actin and JUNB mRNA distribution correspond to the number of ribosomes that translate each molecule.