(A) Map of the human MBP gene and the positively acting cis-elements of MBP P2 promoter. The structure of the MBP gene was based on the study of Li et al.29 (B) Identification of the transcriptional start site of the MBP gene (P2). Ten micrograms of poly(A)⁺ RNA from YJ-11 cells was subjected to primer extension and the resulting radiolabeled DNA product was electrophoresed on a 6% denaturing polyacrylamide gel next to M13mp18 DNA-sequence size markers.