Total RNA was used from four developmental stages (two cells, dome/30% epiboly, 14 somites, and prim-6) and extracted using TRIzol (Invitrogen) according to the manufacturer's instructions and used for subsequent RNA-seq based profiling. The RNA samples were treated with 2U DNase I (Qiagen) per μg RNA sample at 37°C for 10 min. Digested samples were then treated with 20 mg/mL proteinase K (Sigma Aldrich) at 37°C for 45 min. The quality and quantity of total RNA were assessed with the Bioanalyzer 2100 (Agilent). The RNA-seq library was generated following the standard Illumina RNA-seq poly(A)+ protocol and sequenced with 76 bp Paired End reads using an Illumina Genome Analyzer IIx at the Barts and The London Genome Center.