“…Neuronal fate is determined by the timing of progenitor cell cycle exit, and the competence of progenitors to generate deep-layer neurons decreases over time (Luskin et al, 1988;McConnell and Kaznowski, 1991;Desai and McConnell, 2000;Frantz and McConnell, 1996). In vitro studies reveal that cortical neurons differentiate in an ordered fashion, with deep-layer neurons born first followed by more superficial ones, consistent with intrinsic clock-like mechanisms of differentiation (Gaspard et al, 2008;Qian et al, 2000;Shen et al, 2006); however, extrinsic factors can influence the time of cell cycle exit and are thus important regulators of cortical neuronal diversity (Mizutani and Saito, 2005;Rodriguez et al, 2012). Nevertheless, the control of progenitor cell cycle exit is unlikely to be the sole factor that defines the fates of cortical neurons.…”