Gene Expression Patterns 7 (2007) 215–226 www.elsevier.com/locate/modgep 1567-133X/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.modgep.2006.10.004 lyl-1 and tal-1/scl, two genes encoding closely related bHLH transcription factors, display highly overlapping expression patterns during cardiovascular and hematopoietic ontogeny Sébastien Giroux a,b,c,1 , Anna-Lila Kaushik a,b,c,1 , Claude Capron a,b,c,d , Ali Jalil b,c,e , Charikleia Kelaidi a,b,c , Fred Sablitzky f , Dominique Dumenil g , Olivier Albagli a,b,c , Isabelle Godin a,b,c,¤ a INSERM U790, Institut Gustave Roussy-PR1, 39 Rue Camille Desmoulins, 94805 Villejuif, France b Institut Gustave Roussy, 39 Rue Camille Desmoulins, 94805 Villejuif, France c Université de Paris XI, Orsay, France d Laboratoire d’Hématologie et d’Immunologie, Faculté de Médecine de Versailles-Paris Ouest, Garches, France e Service Commun de Microscopie Confocale, Institut Gustave Roussy, 39 rue Camille Desmoulins, 94805 Villejuif, France f The University of Nottingham, School of Biology, Institute of Genetics, Queen’s Medical Centre, Nottingham NG7 2UH, United Kingdom g Institut Cochin, Département d’Hématologie, Bâtiment Gustave Roussy, 27 rue du Faubourg St Jacques, 75014 Paris, France Received 17 July 2006; received in revised form 29 September 2006; accepted 6 October 2006 Available online 11 October 2006 Abstract The TAL-1/SCL and LYL-1 genes encode two closely related basic helix–loop–helix transcription factors involved in child T-acute lymphoblastic leukemia through chromosomal rearrangements and transcriptional deregulation. During ontogeny, Tal-1/SCL is required for hematopoietic cell generation, both in the yolk sac, where erythro-myeloid cells are Wrst produced, then in the intra-embryonic com- partment, where hematopoietic stem cells independently arise. We describe here the expression pattern of lyl-1 in mouse embryos from 7 to 14 days post coitus using in situ hybridization, as well as -Galactosidase (-Gal) expression in lyl-1-lacZ knock-in embryos, which express a C-terminally truncated Lyl-1 protein fused to the -Galactosidase (Lyl-1/-Gal). In addition, we compare lyl-1 expression pat- tern with that of tal-1/scl. Similar to Tal-1/SCL, Lyl-1 mRNA expression occurs in the developing cardiovascular and hematopoietic sys- tems. However, contrary to tal-1/scl, lyl-1 is not expressed in the developing nervous system. In lyl-1-lacZ knock-in heterozygous and homozygous embryos, -Gal expression completely correlates with Lyl-1 mRNA expression in the intra-embryonic compartment and is present: (1) in the developing hematopoietic system, precisely where hematopoietic stem cells emerge, and thereafter in the fetal liver; (2) in the developing vascular system; and (3) in the endocardium. In contrast, whereas Lyl-1 mRNA is expressed in yolk sac-derived endo- thelial and hematopoietic cells, Lyl-1/-Gal is either absent or poorly expressed in these cell types, thus diVering from Tal-1/SCL, which is highly expressed there at both mRNA and protein levels.  2006 Elsevier B.V. All rights reserved. Keywords: AGM; Aorta; Aortic clusters; bHLH; Cardiovascular; Endocardium; Endothelial cells; Fetal liver; Heart; Hematopoiesis; Hematopoietic stem cell; Lyl-1; Macrophages; Mouse embryo; Sub-aortic patches; Tal-1/SCL; Transcription factor; Yolk sac Abbreviations: AGM, aorta–gonads–mesonephros; dpc, days post-coitum; -Gal, -galactosidase; bHLH, basic helix–loop–helix; FL, fetal Liver; HIAC, hematopoietic intra-aortic clusters; HSC, hematopoietic stem cells; LTR, long-term reconstitution (activity); P-Sp, para-aortic splanchnopleura; S, pairs of somites; SAP, sub-aortic patches; YS, yolk sac. * Corresponding author. Tel.: +33 142 11 41 43; Fax: +33 142 11 52 40. E-mail address: igodin@igr.fr (I. Godin). 1 These authors equally contributed to this work.