Although CD19 is the most ubiquitously expressed protein in normal B-lineage cells, malignant cells rarely lost CD19 antigen during the process of neoplastic transformation and maintain its high-level expression [80% of acute lymphocytic leukemia (ALL), 88% of B cell lymphomas, and 100% of B cell leukemias]. This evidence concerns the gene CD19 and B-cell non-Hodgkin lymphoma.