Immunofluorescent labeling of adult bone marrow cells with anti-runt box antiserum. (A and B) Purified anti-runt box antiserum (anti-rbox) stains the nuclei of bone marrow cells with variable intensity (A). Occasional cells show weak cytoplasmic staining (arrow), but this is also visible with the preimmune serum (pre) (B, arrow). (C and D) Staining with anti-runt box antiserum preincubated with a bacterial extract containing either GST (anti-rbox + GST; C) or GST/runt (anti-rbox + GST/runt; D). The staining is abolished only in the latter sample.