Chromosome studies of human tumors have provided important clues to the location of relevant genes and the mechanisms by which their growth regulatory functions have been altered. In the last decade, molecular dissection of chromosome translocations in hematopoietic tumors and chromosomal deletions in solid tumors have been particularly helpful in demonstrating the involvement of a few previously known oncogenes and a wide variety of previously unknown "cancer genes," which often differ from one type of tumor to another, and normally may either stimulate or inhibit cell growth. These studies have also helped to clarify the importance of multiple genetic lesions in the development of most tumors, and the recently acquired information is already being used in clinical diagnosis and management. In addition, new specific treatments will ultimately result from these findings, but the work to date also clearly indicates that no single, simple answer to human cancer will be forthcoming.