Schematic view of lymphocyte cell development. Developmental stages are depicted as nodes and arrows indicate transition from one stage to another, i.e. specialization. Self-renewing hematopoietic stem cells give rise to T cells in the thymus (green), B cells in the bone marrow (blue) and natural killer cells (NK) via intermediate stages. DN stands for CD4-/CD8-double negative cells, DPL for CD4+/CD8+ double positive large cells, and DPS for CD4+/CD8+ double positive small cells. Cell surface antigens and rearrangement events are partially annotated. The expression data sets investigated in this paper are marked as follows: green ovals for TCell, blue ovals for BCell, and pink boxes for LymphoidTree. We do not investigate developmental stages and transitions depicted in grey.