In Drosophila, dorsoventral polarity is established by the asymmetric positioning of the oocyte nucleus. In egg chambers mutant for cap 'n' collar, the oocyte nucleus migrates correctly from a posterior to an anterior-dorsal position where it remains during stage 9 of oogenesis. However, at the end of stage 9, the nucleus leaves its anterior position and migrates towards the posterior pole. The mislocalisation of the nucleus is accompanied by changes in the microtubule network and a failure to maintain bicoid and oskar mRNAs at the anterior and posterior poles, respectively. gurken mRNA associates with the oocyte nucleus in cap 'n' collar mutants and initially the local secretion of Gurken protein activates the Drosophila EGF receptor in the overlying dorsal follicle cells. However, despite the presence of spatially correct Grk signalling during stage 9, eggs laid by cap 'n' collar females lack dorsoventral polarity. cap 'n' collar mutants, therefore, allow for the study of the influence of Grk signal duration on DV patterning in the follicular epithelium.