Fig. 8. Specification of NB7-3 cell fates by Numb/Notch signaling. GMC terminal divisions in the NB7-3 lineage are asymmetric, with only one daughter cell receiving Numb protein. Daughter cells that receive Numb inactivate Notch signaling, which leads to a neuronal cell fate. The broken green line in EW1 indicates that Numb is not an absolute requirement for this cell fate. In the progeny that do not receive Numb, Notch signaling is maintained. In GMC-1 progeny, Notch signaling induces the differentiation of the GW motoneuron. In GMC-2 and GMC-3 progeny, Notch signaling activates apoptosis. Previous work has shown that Hb is necessary for development of the GMC-1 lineage and that Kr is necessary for development of the GMC-2 lineage (Isshiki et al., 2001; Novotny et al., 2002).