The concept that stem cells are controlled by particular microenvironments known as 'niches' has been widely invoked. But niches have remained largely a theoretical construct because of the difficulty of identifying and manipulating individual stem cells and their surroundings. Technical advances now make it possible to characterize small zones that maintain and control stem cell activity in several organs, including gonads, skin and gut. These studies are beginning to unify our understanding of stem cell regulation at the cellular and molecular levels, and promise to advance efforts to use stem cells therapeutically.

), the authors note that in Fig. 4 A, the staining for Vasa in spindle-E mutant ovaries was incorrect. Consistent with Findley et al. (4), Vasa perinuclear localization is unaffected in spindle-E mutant ovaries. This does not change the major conclusion that the nuage functions to maintain genome stability by repressing the expression of the selfish genetic elements via rasiRNA-mediated gene silencing. Rather, Spindle-E may function at the same hierarchical level or downstream of Vasa to regulate the localization of Aubergine, Krimper, and Maelstrom. The corrected figure and legend appear below. Fig. 4. Nuage foci are mislocalized in the germ-line-specific RNA-silencing component mutants. Ovaries from different mutant flies were immunostained for the nuage components. Homozygous mutant alleles or allelic combinations were used for all the mutants, except for dcr-1, where clonal analysis was employed. (Scale bar: 10 m.) (A) Localization of the nuage components at the perinuclear regions of the germ-line cells reflects a hierarchical assembly. The nuage components, AUB, KRIMP, and MAEL, depend on SPN-E and VAS to localize normally to the perinuclear regions; KRIMP and MAEL depend on SPN-E, VAS, and AUB to localize to the nuage; and MAEL depends on SPN-E, VAS, AUB, and KRIMP to localize normally. (B) Nuage localization is unaffected in the conventional dicing enzyme mutants, dcr-1 and dcr-2. The nuage is an electron-dense perinuclear structure that is known to be a hallmark of animal germ-line cells. Although the conservation of the nuage throughout evolution accentuates its essentiality, its role(s) and the exact mechanism(s) by which it functions in the germ line still remain unknown. Here, we report a nuage component, Krimper (KRIMP), in Drosophila melanogaster and show that it ensures the repression of the selfish genetic elements in the female germ line. The Krimp loss-of-function allele exhibited female sterility, defects in karyosome formation and oocyte polarity, and precocious osk translation. These phenotypes are commonly observed in the other nuage component mutants, vasa (vas) and maelstrom (mael), and the RNAsilencing component mutants, spindle-E (spn-E) and aubergine (aub), suggesting a shared underlying defect that uses RNA silencing. Moreover, we demonstrated that the localization of the nuage components depends on both SPN-E and AUB and that the selfish genetic elements were derepressed to different extents in the nuage component mutants, as well as in aub and armitage (armi) mutants. In the nuage component mutants, vas, krimp, and mael, the levels of roo, I-element, and HeT-A repeat-associated small interfering RNAs were greatly reduced. Hence, our data suggest that the nuage functions as a specialized center that protects the genome in the germ-line cells via gene regulation mediated by repeat-associated small interfering RNAs.Krimper ͉ repeat-associated small interfering RNA