Genome Architecture, Gene Regulation, Stem Cells and Cancer Programme, Centre for Genomic Regulation (CRG), Dr. Aiguader 88, 08003 Barcelona and Universitat Pompeu Fabra (UPF), 08002 Barcelona, Spain Genome Architecture, Gene Regulation, Stem Cells and Cancer Programme, Centre for Genomic Regulation (CRG), Dr. Aiguader 88, 08003 Barcelona and Universitat Pompeu Fabra (UPF), 08002 Barcelona, Spain.