The Pax gene family of nuclear transcription factors is comprised of nine family members that function during embryogenesis to regulate the temporal and position-dependent differentiation of cells. In addition, the family is involved in a variety of signal transduction pathways in the adult organism. Mutations in the Pax family of proteins have been linked to disease and cancer in humans. For example, the human Pax-5 gene encodes a B cell lineage specific protein called B cell specific activator protein or BSAP, which is expressed in pro-B, pre-B and mature B lymphocytes but not in plasma cells. BSAP functions to regulate not only B cell development, but also influences the balance between immunoglobulin secretion and B cell proliferation. Overexpression of BSAP has been implicated in cellular transformation, and in the case of small lyphocytic lymphomas with plasmacytoid differentiation, a t(9;14)(p13;q32) translocation resulting in the deregulation of Pax-5 gene expression has been detected. The gene which encodes Pax-5 maps to human chromosome 9p13.