The LIM-domain-only (LMO) proteins (LMO1-4) are transcriptional regulatory proteins that are not able to directly bind to DNA but rather contain two LIM domains that mediate protein-protein interactions.7, 8, 9, 10 In T-cell acute lymphoblastic leukaemia (T-ALL) cells, either LMO1 or LMO2 is a critical component of a transcriptional complex comprised of TAL1, TCF12/HEB, TCF3/E2A, MYB, RUNX1, GATA3 and LDB1, which forms a positive interconnected auto-regulatory circuit that is a major driver of malignant transformation in ~60% of cases of T-ALL in children and adults.5, 10, 11, 12. The gene discussed is TCF3; the disease is T-cell acute lymphoblastic leukemia.