Figure 2. The role of the Drosophila JAK-STAT pathway in eye development. During the second larval instar, Iro-C genes are induced by Wg signaling in the dorsal domain of the eye disc. Iro-C proteins repress Fng expression such that only ventral cells express Fng. This creates a border of Fng-expressing and non-Fng-expressing cells at the midline of the disc, leading to Notch signaling there. Notch induces expression of upd at the posterior midline of the eye disc. JNK signaling can also induce upd during invasive tumor formation. upd is presumed to be repressed by PRC1 in cells outside of the posterior midline. Upd leads to activation of STAT92E (STAT^P), which represses the Notch ligand Ser to the ventral domain and Wg to the anterior margin. Csk negatively regulates STAT^P, either through Src or through Dome/Hop. Delta (Dl), another Notch ligand in Drosophila, is expressed only in the dorsal domain.