Antonius G. Rolink is Professor of Immunology at the University of Basel. His research focus is the understanding of the molecular mechanisms guiding mouse lymphocyte development. Haematopoietic stem cells (HSCs), a very rare bone marrow cell type, are responsible for the life-long production of all blood cells including T and B lymphocytes. Until recently, it was thought that the differentiation of HSCs into the various haematopoietic cells was a rather hierarchical process with differentiation along a particular lineage associated with a progressive loss of potential of generating other lineages. However, the finding that pro B cells from the transcription factor Pax5 deficient mouse possess the potential to give rise various hematopoietic lineages including myeloid cells have challenged this idea for the first time. Moreover, the recent development of very sensitive and quantitative in vitro assays, together with the identification of new progenitor subpopulations, has challenged this idea even further. Thus, we have identified in the mouse bone marrow a novel population of cells, representing 0.2% of all nucleated cells, which, can differentiate in vitro into B and T lymphocytes, as well as myeloid cells. Based on these findings, we have called these cells “early progenitors with lymphoid and myeloid developmental potential”, or EPLM. Currently we studying the molecular mechanisms that guide the development of EPLM into T and B cells using the various in vitro and in vivo systems that we have established. Yet another research focus of our laboratory is the unraveling of the mechanisms that guide the development of the human hematopoietic system. Therefore we have established a so-called humanized mouse model. Thus, upon transplantation with human CD34+ cord blood stem cells, newborn RAG-2/gc mice show a robust and long-lasting reconstitution of the primary and secondary lymphoid organs by progenitor and mature human lymphocytes. Currently, we are using this model system to analyze the molecular mechanisms underlying human lymphocyte development. Our group is also interested the mechanisms that guide regulatory T cell formation and moreover we analyze the therapeutic potential of these cells in various disease models.