The transcription factor Pax5 is essential for commitment of lymphoid progenitors to the B lymphocyte lineage. Pax5 fulfils a dual role by repressing B lineage 'inappropriate' genes and simultaneously activating B lineage–specific genes. This transcriptional reprogramming restricts the broad signaling capacity of uncommitted progenitors to the B cell pathway, regulates cell adhesion and migration, induces V_H-DJ_H recombination, facilitates (pre-)B cell receptor signaling and promotes development to the mature B cell stage. Conditional Pax5 inactivation in early and late B lymphocytes revealed an essential role for Pax5 in controlling the identity and function of B cells throughout B lymphopoiesis. PAX5 has also been implicated in human B cell malignancies, as it is deregulated by chromosomal translocations in a subset of acute lymphoblastic leukemias and non-Hodgkin lymphomas.