Model of Zac1 function in the retina. Zac1 negatively regulates amacrine cell number cell non-autonomously, controlling TGFβII expression, which inhibits amacrine cell genesis at threshold levels (negative feedback). In contrast, Zac1 negatively regulates rod number cell-autonomously. Zac1 negatively regulates (either directly or indirectly) the expression of genes involved in the specification/differentiation of an amacrine cell (Pax6, Six3, Foxn4, Math3, NeuroD, Bhlhb5, Barhl2) and rod cell (Rb, Otx2, Crx, Nr2e3, Nrl) fate by controlling the decision by retinal progenitor cells to differentiate along these lineages. AC, amacrine cell; APC, amacrine precursor cell; PPC, photoreceptor precursor cell; RPC, retinal progenitor cell.