The structure and function of the immunoglobulin and T-cell receptor genes are reviewed and discussed in relation to clonality in hematologic malignancies and the use of laboratory analysis of DNA gene structure studies of B- and T-cell malignancies. The applications of molecular diagnostic techniques in hematologic disease, including lymphomas and leukemias, are reviewed. The use of restriction fragment length polymorphism (RFLP) analysis for bone marrow transplant patients and oncogene analysis in the diagnosis of hematologic malignancies is discussed. Polymerase chain reaction (PCR) techniques used to evaluate the BCR/abl gene fusion product in chronic myelogenous leukemia (CML) are briefly covered.