A model of Eya1 and SHH regulation of symmetric division for GCPs. During development, SHH signaling is highly active, thereby promoting symmetric division. Eya1 de-phosphorylates aPKC, which leads to reduced phosphorylation of Numb and equal distribution of Numb to the two daughter cells. Therefore, the daughter cells both remain in the EGL and re-enter the cell cycle. As development proceeds, Shh and Eya1 expression decrease, and therefore GCPs divide asymmetrically, producing one daughter cell that exits the cell cycle and migrates to the IGL, and one that reenters the mitotic cycle.