Deletions in the short arm of chromosome 3 have long been known to be common in many tumor types, including carcinomas of the lung and kidney. Small interstitial deletions of the proximal-central region of 3p, with band 3p14 as a minimal common deleted segment, have recently been shown to occur in as many as 10% of carcinomas of the breast, often as the only chromosomal change. Seemingly identical deletions may also be found in the epithelial cells of mixed-lineage benign tumors of the breast and even in diffuse proliferative breast disease, a disorder that would not normally be accepted as neoplastic, but never in completely normal breast tissue. The cytogenetic evidence therefore indicates that the putative tumor suppressor gene deleted from 3p14 influences cellular proliferation; evidently, its loss is often not sufficient for a fully malignant phenotype to emerge. The first information about FHIT, a candidate suppressor gene recently identified in the FRA3B fragile site in 3p14 and found to be abnormal or lost in a high percentage of carcinomas of various organs, including breast, is compatible with such a general proliferation-regulating role.