(#1) splice acceptor (SA) signals at the 3’ end of each intron and the first nucleotide of the downstream exon, almost always N-C-A-G-{cut}-G

(#2) a “polypyrimidine” tract 20-50 nucleotides upstream of the SA AG

(#3) a “branch point” C-U-R-{A}-Y upstream of the polypyrimidine tract; this is where the 5’ end of the intron joins in an unsual 5’-2’ phosphodiester bond to the {A} to form the excised intron “lariat” structure;

(#4) splice donor (SD) signals at the 5’ end of each intron, almost always GU; the SD includes sequence elements from the upstream exon and the intron (M-A-G-{cut}-G-U-R-A-G-U).

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