In order to establish the relationship between ALL cells in the BM and in the PB, and to resolve how the anatomical location is reflected in the overall gene expression pattern of a pre-B ALL cell, we developed a purification approach based on the presumption that the transformed cells express the lineage marker CD19, but due to the developmental block lack the expression of Immunoglobulin light chain (IgL) protein, normally not expressed until later stages of development [7], on the cell surface. The gene discussed is CD19; the disease is acute lymphoblastic leukemia.