It is from an analysis of these data that I proposed that losses of all or part of the long arm of chromosomes 5 and/or 7 may be indicators of exposure to mutagenic agents (8). Moreover, our analysis of the regions of chromosomes 5 and 7 that are consistently missing can define the critical region of each chromosome with some precision. Looked at from another perspective, the frequency of losses of chromosomes 5 and/or 7 may be an indicator of the proportion of ANLL in each particular population that is related to some mutagenic exposure. These aberrations are most frequent in cells of patients who were previously exposed to various cytotoxic regimens for treatment of a primary (usually malignant) disease; these patients are considered to have 2 degrees ANLL. Among patients with ANLL de novo, abnormalities of chromosomes 5 and 7 are much more frequent in older than younger patients. Finally, among adult patients with ANLL de novo, -5 and -7 are more common in those whose occupations could potentially expose them to mutagenic agents such as chemicals, pesticides, or petroleum products. With regard to chromosome 5, most of these deletions are interstitial and always include 5q23 through q31, which I have called the critical region. Although I am less certain with regard to chromosome 7, it appears that the critical region that is consistently deleted may be 7q32 or 7q34-35.