“…In particular, specification and differentiation of CSN is regulated first by controls over broad corticofugal projection neuron differentiation, then by controls over all subcerebral projection neurons, and then by CSN-subtype-specific molecular controls ( Arlotta et al, 2005 ; Cederquist et al, 2013 ; Chen et al, 2005a , 2005b ; Galazo et al, 2016 ; Greig et al, 2016 ; Han et al, 2011 ; Joshi et al, 2008 ; Kwan et al, 2008 ; Lai et al, 2008 ; Lodato et al, 2014 ; McKenna et al, 2011 ; Molyneaux et al, 2005 ; Ozdinler and Macklis, 2006 ; Shim et al, 2012 ; Tomassy et al, 2010 ; Woodworth et al, 2012 , 2016 ). Guidance molecules direct CSN axons at specific choice points, including in the spinal cord (e.g., EphA4; reviewed by Canty and Murphy 2008 ; Sahni et al 2020 ; Welniarz et al 2017 ). Further, specific signaling mechanisms promote CSN axon extension into the cord (e.g., insulin growth factor [IGF]-I [ Ozdinler and Macklis, 2006 ] and Wnt-Ryk signaling [ Liu et al, 2005 ]).…”