IL-6 expression of clones A4 (red) and D8 (green) of NRP-154 cells. Clones of NRP-154 cells were derived by limit dilution at 1 cell/well; calculated cloning efficiency was 17.8%. Clones were fixed, permeabilized, and stained with biotinylated anti-IL-6 (R&D Systems) plus streptavidin-phycoerythrin (Pharmingen), as described in Materials & Methods. Fluorescence in FL2 was analyzed on a FACScan. The IL-6 expression of the 16 clones was either high, like A4 (shown in red), or low, like D8 (shown in green). A4 cells are 8 times as fluorescent as D8 cells. The IL-6 expression of 2 of the 16 clones is shown for the sake of clarity. The fluorescence intensity of clone A4 was 10× that of clone D8 in this experiment (p < 0.001 by KS statistics). A histogram from 1 determination is shown here.