SF3B1 and cancer: Aberrant splice sites were located within a narrow window of 15–24 nucleotides (nt) upstream from the canonical 3′ ss (Figure 3b) and associated with sequence features, including a shorter polypyrimidine tract and an enrichment of adenines at positions -8 to -18 upstream of the cryptic 3′ ss (Figures 3c and d), as described for human cancers with SF3B1 mutations.13 Since SF3B1 is known to play a major role in U2 snRNP recruitment to the splicing branchpoint (BP), we performed lariat sequencing to identify BPs in aberrantly spliced mouse mRNAs.