PAX5 is a transcription factor essential for the commitment of lymphoid progenitor cells to the B-lymphocyte lineage. PAX5 fulfills a kind of dual role by repressing "inappropriate" B-lineage genes and simultaneously activating B-lineage-specific genes. This transcriptional reprogramming restricts the broad signaling capacity of unattached progenitors to the B-cell signaling pathway, regulates cell adhesion and migration, induces VH-DJH recombination, facilitates (pre)B-cell receptor signaling, and promotes development to the mature B-cell stage.
Inactivation of PAX5 in early and late B lymphocytes demonstrated that PAX5 plays an essential role in controlling B cell identity and function throughout B lymphopoiesis. PAX5 has also been demonstrated in human B-cell malignancies, being deregulated by chromosomal translocations in a subset of acute lymphoblastic leukemias and non-Hodgkin lymphomas.
Immunohistologically, PAX5 functions as a nuclear marker for B lymphocytes and is an alternative to CD20 and CD79a.