We generated a guide RNA (gRNA) library targeting the 60 splicing factor genes upregulated following Apc deletion (Supplementary Data 2 and 6) and delivered them by lentiviral transduction into either wild-type-Cas9 or Apcfl/fl-Cas9 intestinal organoids (Fig. 1h) at low multiplicity of infection (MOI). Here, APC is linked to infection.