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Single-letter abbreviations for the amino acid residues are as follows: A, Ala; C, Cys; D, Asp; E, Glu; F, Phe; G, Gly; H, His; I, Ile; K, Lys; L, Leu; M, Met; N, Asn; P, Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; W, Trp; and Y, Tyr. X indicates any residue. 30. We thank P. Chambon for critical reading of the manuscript and for providing nuclear receptor expression vectors, H. Gronemeyer for providing TIF2 expression vectors, R. H. Goodman for the CBP cDNA, and S. Hanazawa and A. Takeshita for helpful discussion.