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Cite as: C. Charenton et al., Science 10.1126/science.aax3289 (2019). RESEARCH ARTICLES First release: 11 April 2019 www.sciencemag.org (Page numbers not final at time of first release) 1 Pre-mRNA splicing consists of removal of non-coding introns and ligation of coding exons by two transesterification reac-tions catalyzed by an immense ribonucleoprotein particle called the spliceosome. First, the pre-mRNA 5′ splice site (5′SS) is attacked by the conserved branch-point (BP) aden-osine upstream of the 3′SS, yielding the cleaved 5′-exon and lariat-intron/3′-exon intermediate. Then, the freed 3′ hy-droxyl of the 5′-exon attacks the 3′SS, ligating the exons and releasing the lariat intron (1–3). The spliceosome assembles in a stepwise manner from five snRNPs, each containing an snRNA (U1, U2, U4, U5 or U6) and then undergoes an extensive remodeling process to form its active site (1–5). The accuracy of pre-mRNA splicing relies on splice site recognition by the spliceosome, which is mainly ...