Table 1.

Purine-rich sequences tend to be present not only in the alternative exon of a pre-mRNA but also in flanking exons

Enhancer-containing exon	Purine-rich sequences in flanking exon
IgM exon M2	Upstream exon M1: (17/116) gctGAGGAGGAAGGctt
BGH exon 5	Upstream exon 4: (20/162) tatGAGAAGctGAAGGAcct
ASLV env	Distal 5′ exon (23/379) gttGGAAGAcGGGAAGGAAGccc
dsx female	Upstream exon 3: (15/139) ggcGAAtcGAAGAGg/gu
Fibronectin EDIIIA	Upstream exon −1: (25/114) ctcAGAAtccAAGcGGAGAGAGtca
	Downstream exon +1: (16/118) cccAAGGAGAAGAccg
Caldesmon exon 5	Downstream exon 6: (20/78) ag/gGGAGAAGAGAAGGGAActa

The cDNA and genomic sequences of representative mRNAs containing well characterized exonic enhancers were surveyed for additional purine-rich exonic sequences (38–44). Shown are purine-rich sequences found in exons that flank enhancer-containing exons. The numbers in parentheses are the number of exonic nucleotides shown per the number of nucleotides in the exon. Purine residues are in capital letters. Points where sequences extend into introns are marked by a slash.