Focus of the research in the MID research group has always been on the interface between normal, dysregulated and malignant lymphoid development. One major aspect of our work is around V(D)J recombination of immunoglobulin (IG) and T-cell receptor (TR) genes, which is a key process during (early) lymphoid differentiation to establish a broad repertoire of antigen-recognizing receptors. In (chronic) lymphoid malignancies the exact IG and TR composition is thought to -at least partly- reflect antigenic stimulation, which may play a role in pathogenesis. Although the V(D)J recombination process is tightly regulated, aberrant V(D)J recombination occurs, resulting in the coupling of IG-TR loci to oncogenes that are transcriptionally deregulated, eventually resulting in a block in lymphoid differentiation. Insight into IG-TR recombination events will thus shed light on the pathogenic mechanisms underlying leukemia formation. Another major aspect of our research is on immune ageing of lymphoid cells (both T and B cells) and the potential relationship with lymphoid malignancies in elderly, such as chronic lymphocytic leukemia (CLL) and large granular lymphocytic (LGL) leukemia.