Primer extension analysis of transcripts derived from the rat CBP locus. RNA was isolated from Rin-m cells and primer extension analysis was performed based on TSSs determined by cap-trapping. Products were resolved by denaturing polyacrylamide gel electrophoresis. Marker (M): End labeled Φ X174 Hinf 1 digested DNA. 5', internal, and 3' refer to the position of the TFIIB GST and TSS relative to CBP gene boundaries.