Pax-5 is a B-cell-specific activator protein (BSAP). In the early stages of B cell development, Pax-5 influences the expression of several B-cell-specific genes, such as CD19 and CD20. Pax-5 is expressed primarily in pro-, pre-, and mature B cells, but not in plasma cells. Interestingly, Pax-5 mRNA is transiently detected in the mesencephalon and spinal cord during embryogenesis. Expression then shifts to the fetal liver and correlates with the onset of B lymphopoiesis. This indicates that Pax-5 is important in B cell development, but may also have a role in proper neuronal development.