Asymmetric cell division is a widespread mechanism in developing tissues that 
leads to the generation of cell diversity. In the embryonic central nervous 
system of Drosophila melanogaster, secondary precursor cells-ganglion mother 
cells (GMCs)-divide and produce postmitotic neurons that take on different cell 
fates. In this study, we show that binary fate decision of two pairs of sibling 
neurons is accomplished through the interplay of Notch (N) signaling and the 
intrinsic fate determinant Numb. We show that GMCs have apical-basal polarity 
and Numb localization and the orientation of division are coordinated to 
segregate Numb to only one sibling cell. The correct positioning of Numb and the 
proper orientation of division require Inscuteable (Insc). Loss of insc results 
in the generation of equivalent sibling cells. Our results provide evidence that 
sibling neuron fate decision is nonstochastic and normally depends on the 
presence of Numb in one of the two siblings. Moreover, our data suggest that the 
fate of some sibling neurons may be regulated by signals that do not require 
lateral interaction between the sibling cells.