a, Representative immunofluorescence images of HA–RREB1 in SMAD4-restored PDA cells treated with DMSO or 1 μM ERK inhibitor SCH772984 (ERKi) for 6 h. Scale bar, 20 μm. Data are representative of two independent experiments. b, c, Western blot analysis of RREB1 (b) or HA–RREB1 levels (c) in SMAD4-restored PDA cells treated with DMSO, 1 μM ERKi or 1 μM AZD6244 (MEKi; an inhibitor of the ERK-activating kinases MEK1/2) for the indicated time periods. Tubulin immunoblotting was used as loading control. Data are representative of two independent experiments. d, ChIP–PCR analysis of HA–RREB1 binding to the indicated sites (Figs. 1g, 3b) in Snai1, Has2 and Il11 in SMAD4-restored PDA cells that were treated with vehicle (DMSO) or ERKi (1 μM) for 6 h. Mean ± s.e.m. n = 3, two-way ANOVA. **P < 0.01, ***P < 0.001, ****P < 0.0001. e, SMAD4-restored PDA cells expressing HA–RREB1 were treated with ERKi for the indicated length of time. HA–REBB1 was tested for binding to Snai1 DE2 and Has2 PP1 double-stranded DNA oligonucleotide probes in DNA affinity precipitation assays. Data are representative of two independent experiments. f, Schematic of RREB1. Each tick represents a previously annotated phosphorylation site in PhosphoSitePlus identified in at least two independent mass spectrometry experiments. Red filled circles represent high stoichiometry (>15%) phosphorylation sites that are inhibited by ERKi, as identified in g. Zinc-finger domains annotated in Uniprot are shown. g, Phosphorylation stoichiometry of four ERK-dependent RREB1 phosphorylation sites in SMAD4-restored PDA cells, as determined by SILAC mass spectrometry of cells treated with DMSO (control) in light medium or ERKi in heavy medium for 6 h. h, Summary of ERK-dependent RREB1 phosphorylation sites, sequence motifs and phosphorylation stoichiometry. i, Rreb1-KO PDA cells were transduced with the indicated RREB1-WT or phosphorylation-site mutant constructs, then treated with SB505124 or TGF-β for 1.5 h. mRNA levels of Snai1 and Has2 were determined by qPCR with reverse transcription. Mean ± s.e.m. n = 4, two-way ANOVA. ***P < 0.001, ****P < 0.0001. j, ChIP–PCR analysis of HA–RREB1 binding to the indicated sites in Rreb1-KO PDA cells transduced with the indicated RREB1-WT or phosphorylation-site mutant constructs. Mean ± s.e.m. n = 4, two-tailed unpaired t-test. ****P < 0.0001.