Reactivity of the anti-runt box antiserum against runt box polypeptides. (A) Schematic diagram of the different polypeptides tested. The runt box is in black; the chromosome 8 sequence in pBEH/RM is indicated by the hatched box; a short sequence of chromosome 8 origin is also present at the 3′ end of the runt box in plamid pBN-TG2. Numbers refer to amino acids. The predicted molecular weight is on the right. (B) Western blotting analysis of COS-7 cells transiently transfected with plasmids encoding the described runt box polypeptides. “pre” and “anti-rbox” refer to the use of preimmune serum or of affinity-purified anti-runt box antiserum, respectively. As a further specificity check, the pBEH-RM5 lane was split into two and probed with purified anti-runt box antiserum either mock-depleted (by preincubation with matrix-bound GST, “− rbox”) or depleted of anti-GST/runt box reactivity (by preincubation with matrix-bound GST/runt box, “+ rbox”). mock, mock-transfected COS cells. The three leftmost lanes were exposed for 18 hours, the others for 3 days.