blm mutant adults are predisposed to early tumor development. (a) Transverse section through midgut of wild-type fly. Normal, uniform-appearing villi (V) protrude into the gut lumen (L). A cluster of bacteria and yeasts (normal gut flora) is present within the lumen. (b) Tumor within the midgut of a blm mutant fly. A dense cluster of small tumor cells (T) arises from within the mucosa and forms an irregular mass that protrudes into the gut lumen. (c) Normal testis from a wild-type fly. Normal spermatocytes (S) in various stages of development are present. (d) Tumor within the testis of a blm mutant fly. Large numbers of small tumor cells (T) resembling early germ line precursors have replaced normal spermatocytes within the testis. (e) Normal maturing ovarian follicles from a wild-type fly. The largest (arrowhead) contains normal cyst cells surrounded by a single layer of follicular cells. (f) Ovarian tumor from a blm mutant fly. Large numbers of tumor cells (T) resembling immature germ line precursor cells replace normal cystocytes. All sections were prepared from 35-day-old flies and were stained with hematoxylin and eosin. Each scale bar = 50 microns. (g) Histogram showing tumor frequency in 35- to 60-day-old adult flies. *P < 0.05 compared to wild type (WT; two-tailed Fisher's exact test), **P < 0.01.