THY1 and fragile X syndrome: Most in vivo imaging studies of spine dynamics in FXS focus on the apical dendrites of L5 PyrNs, leveraging the sparse but very bright neuronal labeling conveniently offered by the Thy1-YFP-H or Thy1-GFP-M line (Pan et al., 2010; Padmashri et al., 2013; Nagaoka et al., 2016; Hodges et al., 2017).