In this review, we discuss the structure, organization and activation of the immunoglobulin (Ig) and T cell receptor (TCR) genes and the use of DNA probes derived from these genes as tools to determine clonality and cell lineage in hemopoietic malignancies. We also examined briefly the participation of immunoglobulin and TCR genes in chromosomal aberrations present in many lymphoproliferative disorders, often involving protooncogenes and potentially of tumorigenic consequences.