Several important human syndromes provide evidence for the idea that the rate of gene mutation and chromosome breakage is under genetic control. Although not understood in detail, their underlying mechanisms include deficiencies in DNA replication, repair, and increased sensitivity to external agents. The ease of genetic analysis in Drosophila melanogaster offers a number of model systems that may give useful insights into comparable human conditions. For example, some of the many meiotic mutants isolated from natural populations provide information on the genetics of DNA repair in a eukaryote. In addition, complex syndromes affecting mutation rate are available for study. One such system is hybrid dysgenesis in Drosophila. Hybrid dysgenesis refers to a collection of genetic changes, including chromosome breakage, increased mutation rates, and sterility expressed in crosses between independent population lines. Our recent work has focused upon chromosome breakage and mutator activity in this system. The pattern of inheritance and expression of mutator activity implicates a chromosome-cytoplasm interaction for its induction. Furthermore, the genetic elements responsible for this phenomenon appear to move from one location or chromosome to another. Indeed, one can draw a convincing parallel between hybrid dysgenesis and the behavior of transposable, or nomadic, DNA sequences. Thus, one must view the phenotype "mutation rate" as a conglomerate of factors affecting DNA stability, physiological condition, and the behavior of genetic elements.