The establishment of polarity along the dorsal-ventral axis of the Drosophila embryo requires the graded distribution of the dorsal morphogen. Several maternal genes are responsible for the formation of the gradient and their products act in an ordered series of events that begins during oogenesis and involves two different cell types, the oocyte and the follicle cells. The last step in the series results in selective nuclear localization of dorsal proteins, dorsal is thought to regulate the expression of zygotic genes in a concentration dependent way. The zygotic genes determine cell fates in specific regions of the embryo and direct other genes involved in the processes of differentiation.