library_construction_protocol: HH4 embryos were bilateraly injected with control and target (Wnt) morpholinos. Neural folds from morpholino injected side were dissected from HH9 embryos (n=5 per ATAC assay). Cells were dissociated in Accumax for 20min at RT under mild agitation. ATAC-seq library preparation was performed as previously described (Buenrostro et al., 2015) and sequenced with paired-end reads on an Illumina NextSeq500 instrument.