The timing of cortical neurogenesis has a major effect on the size and organization of the mature cortex. The deletion of the LIMhomeodomain transcription factor Lhx2 in cortical progenitors by Nestin-cre leads to a dramatically smaller cortex. Here we report that Lhx2 regulates the cortex size by maintaining the cortical progenitor proliferation and delaying the initiation of neurogenesis. The loss of Lhx2 in cortical progenitors results in precocious radial glia differentiation and a temporal shift of cortical neurogenesis. We further investigated the underlying mechanisms at play and demonstrated that in the absence of Lhx2, the Wnt/β-catenin pathway failed to maintain progenitor proliferation. We developed and applied a mathematical model that reveals how precocious neurogenesis affected cortical surface and thickness. Thus, we concluded that Lhx2 is required for β-catenin function in maintaining cortical progenitor proliferation and controls the timing of cortical neurogenesis.cortical neurogenesis | Lhx2 | β-catenin U nderstanding how genetic mechanisms interact to set up a precise developmental timing is a fundamental issue in biology. In the cerebral cortex, excitatory neurons are generated by progenitor cells in the dorsal telencephalon (dTel) lining the lateral ventricle. During the early developmental stages, cortical progenitors undergo symmetric divisions, resulting in the proliferation of progenitors and thereby allowing expansion of the developing cortex. Soon after, cortical progenitors start generating distinct types of neurons through asymmetric differentiative divisions (1-5). The precise timing of the switch from proliferative division to differentiative division is crucial to determining the number of cortical neurons, and thus the cortical size.The switch from proliferation to differentiation is reportedly regulated by the canonical Wnt signaling pathway, in which β-catenin (β-Cat) is the major downstream effector. In the absence of Wnt signaling, β-Cat is phosphorylated by glycogen synthase kinase 3 and targeted for proteosome degradation. Once Wnt ligands bind to the Frizzled-Lrp5/6 receptors, the activity of glycogen synthase kinase 3-Axin-APC (adenomatous polyposis coli) destruction complex is inhibited. As a consequence, β-Cat accumulates in the cytoplasm, translocates to the nucleus, and activates downstream gene transcription together with the lymphoid enhancer-binding factor (LEF)/ T-cell factor (TCF) transcription factors (6). Overexpression of the stabilized, N-terminally truncated form of β-Cat in cortical progenitors during early neurogenesis promotes their overproliferation (7,8), whereas inactivation of β-Cat in the cortex promotes neurogenesis (9, 10). However, stabilized β-Cat was also shown to promote cortical progenitor differentiation (11). Thus, it has been proposed that Wnt/β-Cat signaling promotes proliferation and differentiation of cortical progenitors at early and late developmental stages, respectively (12). This raises the essential and largely open question of ho...