Immunohistochemistry aids in diagnosis, including positive expression of CD19, CD20, CD22, CD10, and CD79a, whereas CD5, CD23, Bcl-2, and TdT are usually negative, assisting in differentiating PCNSBL from other lymphomas and leukemia subtypes.[21] Molecular genetic testing, particularly the rearrangement status of the Myc gene, provides critical diagnostic information for PCNSBL. The gene discussed is CD19; the disease is leukemia.