The M3-3D9 (SN1a) monoclonal antibody specifically recognizes CD231 which is also known as T-cell acute lymphoblastic leukemia-associated antigen 1 (TALLA-1) or Membrane component chromosome X surface marker 1 (MXS1). CD231 (TALLA-1) consists of four transmembrane domains, a short extracellular loop, a short intracellular loop, a longer extracellular loop, and short N- and C-terminal cytoplasmic tails. This ~150 kDa multi-pass transmembrane glycoprotein is encoded by TSPAN7 (Tetraspanin 7). It belongs to the tetraspanin superfamily (TM4SF) and is likewise known as Transmembrane 4 superfamily member 2 (TM4SF2). CD231 (TALLA-1) may play roles in integrin-mediated migration and the control of neurite outgrowth. It is expressed on the surface of T-cell type acute lymphoblastic leukemia (T-ALL) cells, neuroblastoma cells, and normal brain neurons. CD231 (TALLA-1) gene mutations are associated with X-linked mental retardation.