Ldb1, a ubiquitously expressed LIM domain binding protein, is essential in a 
number of tissues during development. It interacts with Gata1, Tal1, E2A and 
Lmo2 to form a transcription factor complex regulating late erythroid genes. We 
identify a number of novel Ldb1 interacting proteins in erythroleukaemic cells, 
in particular the repressor protein Eto-2 (and its family member Mtgr1), the 
cyclin-dependent kinase Cdk9, and the bridging factor Lmo4. MO-mediated 
knockdowns in zebrafish show these factors to be essential for definitive 
haematopoiesis. In accordance with the zebrafish results these factors are 
coexpressed in prehaematopoietic cells of the early mouse embryo, although we 
originally identified the complex in late erythroid cells. Based on the change 
in subcellullar localisation of Eto-2 we postulate that it plays a central role 
in the transition from the migration and expansion phase of the 
prehaematopoietic cells to the establishment of definitive haematopoietic stem 
cells.