The somatic mutation theory of tumorigenesis states that mutations are necessary for tumor development. On the other hand, acquired, clonal chromosomal alterations are occasionally detected in otherwise normal, nonneoplastic cells—for example, loss of sex chromosomes occurs in bone marrow cells and lymphocytes in elderly individuals—and it is therefore evident that not all mutations are by themselves sufficient for neoplasia to occur. Thus, the finding of an acquired, clonal chromosomal abnormality does not constitute proof that a lesion is neoplastic. Trisomy 7 has, as the sole clonal chromosomal aberration, been reported in a wide variety of epithelial tumor types but also in some mesenchymal and neurogenic neoplasms. It has been suggested to be a primary, i.e., tumor-initiating, abnormality in tumors of the bladder, brain, colon, kidney, lung, ovary, prostate, and thyroid. But data from cytogenetic studies of solid tumors, macroscopically normal tissue in the proximity of solid tumors, and nonneoplastic lesions now question the importance of a solitary +7 as a neoplasia-associated change. Most solid tumors in which trisomy 7 has been found as the sole change in one clone have also displayed other, cytogenetically unrelated, clones with complex karyotypic abnormalities. Such karyotypic differences among coexisting clones could indicate that the neoplasm is polyclonal, that the cytogenetically disparate clones have emerged during tumor progression from one original clone carrying submicroscopic genomic changes only, or that the clone with +7 does not represent the tumor parenchyma. The latter interpretation is supported by the finding of cells with trisomy 7 in macroscopically normal tissue outside tumors of the brain, kidney, and lung. A seemingly decisive argument against the belief that the finding of an acquired, clonal +7 proves that a neoplasm exists is the detection of clones with an extra copy of chromosome 7 in several nonneoplastic lesions and tissues, such as atherosclerotic plaques, chorionic villi, chronic pyelonephritis, Dupuytren's contracture, focal steatosis of the liver, Peyronie's disease, and rheumatoid synovitis. That the abnormality arises in vivo has been shown by in situ hybridization with chromosome 7 specific probes; +7 is no in vitro artifact. It is unknown in which cell type the trisomy occurs; some data from colon adenomas favor epithelial cells, whereas data from kidney tumors and colon carcinomas suggest that the +7 arises in cells of the immune system. This question may possibly be resolved by obtaining a pure cell line with trisomy 7 cells only, to be characterized further by immunologic and morphologic methods. Another line of investigation might be to search for clonal chromosomal abnormalities in nonneoplastic tissues in other species. Finally, it remains to be elucidated whether +7 is a neutral genome mutation or whether it confers a selective advantage upon the cell. © 1993 Wiley-Liss, Inc.