“…Despite its complexity, great strides have been made in understanding the development of cortical heterogeneity. For cortical projection neurons (PNs), heterogeneity has been studied broadly, in the context of areal patterning (Bhaduri et al., 2021; C. H. Chen et al., 2011; Levitt et al., 1997) and lamina identity within a cortical area (R. Luo et al., 2011; Molinard‐Chenu et al., 2020; Qian et al., 2020), as well as more specifically, at the level of a cell subclass—defined by broad neuronal projection differences within a cortical layer (Gerfen et al., 2018; Hatanaka et al., 2016; Kassai et al., 2008; Kast & Levitt, 2019; Molyneaux et al., 2007; Tsyporin et al., 2021)—and, most recently, at the level of a cell type, which further refines subclass identity based on RNA composition (Z. Zhang et al., 2021; E. J. Kim et al., 2020; C. Luo et al., 2017; Zeisel et al., 2015). These efforts have led to a greater understanding of the molecular heterogeneity of mature PNs.…”