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A donor variant (+1, +2 GT→GC, +5, −1) and the four things the spliceosome can do next — skip, cryptic donor, retain, or splice leakily — with frame arithmetic and NMD fate computed for each.

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When the 5′ splice site fails

Normal splicing. U1 snRNA base-pairs with the donor consensus (−3 to +6); the near-invariant GU at +1/+2 is the canonical dinucleotide — near-invariant, not invariant: GC donors exist.

PRE-mRNA · EXONS 2–4 OF 6 · INTRONS COMPRESSEDE2E3E4··· E5–E6EXON 3 DONOR · U1 snRNA PAIRING−3CG−2AU−1GC+1GC+2UA+3AU+4AU+5GC+6UA5′ pre-mRNA3′3′ U1 snRNA5′shaded: canonical +1/+2 · full window: extended consensus −3…+6MATURE mRNA · E1–E6 TO SCALENormal splicingE1E2E3E4E5E6
Where the stop lands decides the allele →