FIGURE 1. Stem-cell decisions. a, A dividing fly neuroblast gives rise to a neuroblast and a smaller ganglion mother cell.

The protein complexes (crescents) at the apical and basal ends of the dividing neuroblast are different, ensuring the asymmetry of division. b, A germline stem cell in the fly ovary, associated with a cap cell, produces a germline stem cell and a cystoblast. The spectrosome (red) anchors one pole of the mitotic spindle to the cap cell to define the orientation of division. c, A germline stem cell in the fly testis, associated with hub cells, produces a daughter germline stem cell and a gonialblast. Yamashita et al.¹ have found that the spectrosome is not involved in this division. d, Yamashita et al. propose that the orientation of division in male germline stem cells is determined by the proteins DE-cadherin (Cad), -catenin, centrosomin (CNN), adenomatous polyposis coli (APC) and an unidentified component, X.