Controlled activation of Receptor Tyrosine Kinases (RTKs) depends on both the presence of the receptor at the cell surface and the availability of its active ligand. In Drosophila, the RTK Torso (Tor) is responsible for the specification of the most anterior and posterior regions of the developing embryo. Tor is distributed ubiquitously on the plasma membrane of the embryo but signals only at the termini due to the localised activation of its ligand Trunk (Trk). While it is understood that Trk activation is mediated by Torso-like (Tsl), a molecule present only at the termini of the early embryo, the mechanism by which it occurs and the precise role of Tsl remains unclear. Currently, it is proposed that Trk must be cleaved in order to bind Tor, and that these proteolytic events are controlled by secretion of Tsl at the embryo poles. However, controversy surrounds these ideas since neither cleaved Trk nor a protease that functions in terminal patterning have been identified. To address this problem we used epitope-tagged forms of Trk to show that Trk is cleaved multiple times in vivo and that these proteolytic events are essential for its function. Unexpectedly, however, we found that Trk cleavage patterns are unaltered in tsl null mutants. These data suggest that Tsl functions post proteolytic processing of Trk to control localised terminal patterning. We are now performing a number of experiments aimed at investigating the precise role of Tsl in the localised activation of Tor signalling.