FIGURE 10-7 Consensus sequences around splice sites in vertebrate pre-mRNAs. The only nearly invariant bases are the 5′ GU and the 3′ AG of the intron (blue), although the flanking bases indicated are found at frequencies higher than expected based on a random distribution. A polypyrimidine tract (hatched area) near the 3′ end of the intron is found in most introns. The branch-point adenosine, also invariant, is usually 20–50 bases from the 3′ splice site. The central region of the intron, which may range from 40 bases to 50 kilobases in length, is generally unnecessary for splicing to occur. See R. A. Padgett et al., 1986, Annu. Rev. Biochem. 55:1119, and E. B. Keller and W. A. Noon, 1984, Proc. Natl. Acad. Sci. USA 81:7417.