Two general features have emerged about genes that are activated after chromosomal translocations in acute forms of cancer. The protein products of these genes are transcription regulators and are involved in developmental processes, and it seems that the subversion of these normal functions accounts for their role in tumorigenesis. The features of the LMO family of genes, which encode LIM-domain proteins involved in T-cell acute leukemia through chromosomal translocations, typify these abnormal functions in tumorigenesis. For example, the LMO2 protein is involved in the formation of multimeric DNA-binding complexes, which may vary in composition at different stages of hematopoiesis and function to control differentiation of specific lineages. In T cells, enforced expression of Lmo2 causes aberrant protein complex formation that primarily seems to hinder the T-cell differentiation program. These observations underscore the conclusion that protein-protein interaction (in this case, through the LIM domain) is a key determinant in tumorigenesis. Furthermore, the study of chromosomal translocations as naturally occurring mutations has been informative about mechanisms in hematopoiesis as well as in tumor etiology.