The ectopic expression of LMO1 or LMO2 in T cell acute leukaemias resulting from chromosomal translocations t(11;14)(p15;qll) or t(11;14)(p13;q11) respectively in a causal factor in tumorigenesis. LMO1 has been found as a heterodimer with a 46 Kd protein in a T cell line derived from a childhood T-acute leukaemia. This 46 Kd protein is the LIM-binding protein LDB1/NLI. The latter is a phosphoprotein and binds to LMO1 in its phosphorylated state and essentially all the LMO1 and LDB1 protein in the T cell line is part of the complex. Therefore, the LMO1-LDB1 interaction is likely to be involved in tumorigenesis after LMO1 is ectopically expressed following chromosomal translocation in T cells prior to development of acute leukaemias.