1K1G:
reference: Structural basis for recognition of the intron branch site RNA by splicing factor 1., Liu Z, Luyten I, Bottomley MJ, Messias AC, Houngninou-Molango S, Sprangers R, Zanier K, Kramer A, Sattler M, Science 2001 Nov 2;294(5544):1098-102. PMID: 11691992

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