The ectopic expression of LMO1 or LMO2 in T cell acute leukaemias resulting from chromosomal translocations t(11;14)(p15;q11) 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.Keywords: leukaemia; dimerization; tumours; chromosomal translocation; transcriptionThe genes encoding the LMO1 and LMO2 proteins (previously known as RBTN1/TTG1 or RBTN2/ TTG2) were discovered as transcription units adjacent to the breakpoints of chromosomal translocations t(11;14)(p15;q11) (Boehm et al., 1988(Boehm et al., , 1990a(Boehm et al., , 1991aMcGuire et al., 1989) and t(11;14)(p13;q11) (Boehm et al., 1991a;Royer-Pokora et al., 1991) respectively. Both LMO1 and LMO2 proteins (as well as the other family member LMO3 (Boehm et al., 1991a;Foroni et al., 1992)) are LIM-only proteins which have two zincbinding LIM domains, each comprising two LIM ®ngers. The chromosomal translocations activating LMO1 and LMO2 are found only in T cell tumours and it seems likely that ectopic expression of the LIMproteins contributes to leukaemogenesis by the ability to form aberrant protein associations in T cells (Rabbitts, 1994). High mRNA and protein levels can be found in T cell lines derived from childhood T cell leukaemias with the relevant translocations (Boehm et al., 1991b;Foroni et al., 1992;Royer-Pokora et al., 1991). Analysis of a T cell line with a t(11;14)(p15;q11), and expressing the LMO1 gene, showed that LMO1 interacts with at least two proteins in the nucleus of these cells; viz. TAL1/SCL and an uncharacterized 46 Kd protein (Valge-Archer et al., 1994). The ability of the LIM domains of LMO1 and LMO2 to act as surfaces for protein interaction (Valge-Archer et al., 1994;Wadman et al., 1994) provides a function for these zinc-containing modules and data on LMO2-containing protein complexes in erythroid cells (Wadman et al., 1994) and in T cell tumours arising in transgenic mice (Grutz et al., 1998) suggest that protein interaction, but not direct DNA binding, is the speci®c function of the LIM-only proteins LMO1 and LMO2.Current molecular models propose that aberrant T cell-associated protein complexes, brought together by the LMO proteins after ectopic expression, contribute to tumorigenesis. The nature of the 46 Kd protein to which LMO1 binds is relevant to this issue because it is present in a complex with LMO1 in human T cell acute leukaemia line carrying the translocation t(11;15)(p15;q11). Thus far, the identity of the 46 Kd protein has not been determined because it is obsc...