Chromosome studies are helping to identify oncogenes, both known and previously unknown, involved in the pathogenesis of human lymphocytic tumors; and mechanisms by which the function of these genes is critically altered. Most extensively studied have been the chromosome translocations involving the myc gene in Burkitt's lymphomas and the bcl-2 gene in low-grade lymphomas, where "activation" of the oncogene results from association with a transcriptionally active immunoglobulin gene. Other putative oncogenes, similarly involved in translocations with immunoglobulin genes (in B-cell tumors) or T-cell receptor genes (in T-cell tumors), are currently being investigated, as well as alternative mechanisms of myc gene activation in these neoplasms. Limited clinical applications of these studies have already been forthcoming, and they should eventually lead to improvements in diagnosis, prognosis, and even therapy.