The 4G7 monoclonal antibody reacts with human CD19, a B cell-specific 95 kDa transmembrane glycoprotein of the immunoglobulin superfamily. CD19 contains two extracellular immunoglobulin-like domains and an extensive cytoplasmic tail. It functions as a positive regulator of B-cell receptor signaling in conjunction with CD21 and CD81. CD19 is highly expressed in most lymphomas and leukemias including some early B-cell malignancies that do not express CD20. For these reasons CD19 is quickly becoming an attractive alternative target for the immunotherapy of lymphoproliferative disorders.