library_construction_protocol: Described in Bonnal et al mRNA sequencing: RNA was isolated using the RNeasy Mini Qiagen kit and the RNA quality was checked using Bioanalyzer (Agilent). Libraries were prepared at the CRG Genomics Unit using the TruSeq stranded mRNA Library Prep (ref. 20020595, Illumina) according to the manufacturer's protocol. Briefly, 500 ng of total RNA were used for polyA-mRNA selection using Oligo-dT beads, and two rounds of purification were performed. During the second elution of the mRNA, this was fragmented under elevated temperature and primed with random hexamers for cDNA synthesis, that was performed using reverse transcriptase (SuperScript II; ref. 18064-014, Invitrogen). Then, a second strand cDNA was synthesized, incorporating dUTP in place of dTTP and generating blunt-ended ds cDNA. A single 'A' nucleotide was added to the 3′ ends of the blunt fragments (A-tailing) and immediately afterwards the Truseq adapter was ligated. Finally, PCR selectively enriched those DNA fragments that had adapter molecules on both ends. The PCR was performed using Unique Dual Indexes and the master mix provided with the kit. All purification steps were performed using AgenCourt AMPure XP beads (ref. A63882, Beckman Coulter). Final libraries were analyzed using Bioanalyzer DNA 1000 or Fragment Analyzer Standard Sensitivity (ref. 5067-1504 or ref. DNF-473, Agilent) to estimate the quantity and validate the size distribution, and were then quantified by qPCR using the KAPA Library Quantification Kit KK4835 (ref. 07960204001, Roche). Amplicon sequencing: Amplicons were checked using Bioanalyzer (Agilent) and the final steps amplification (consisting in the addition of the Illumina adaptors) were performed at CRG Genomics Unit. Both the input and output PCR products were deep sequenced. After quantification by qubit, amplification was performed by PCR using NEBNext Q5 Hot Start HiFi PCR Master Mix (ref. M0543L) and Index 1 (i7) and Index 2 (i5) primers containing Illumina's P5 and P7 sequences. Final libraries were analyzed using Agilent Bioanalyzer or Fragment analyzer High Sensitivity assay (ref. 5067‐4626 or ref. DNF‐474) to estimate the quantity and check size distribution, and were then quantified by qPCR using the KAPA Library Quantification Kit KK4835 (Ref. 07960204001, Roche).