PCP4 and neurodegenerative disease: Several genes were found to be overexpressed in a majority of ExN subclusters (at least five out of eight; Fig. 2C; Table 2), including Calm1, an essential regulator of early neuronal migration (Kobayashi et al., 2015), Fth1, a ferroxidase enzyme that supports iron detoxification as part of the neuronal antioxidant defense system (Mukherjee et al., 2020), Scg5, a secretory chaperone that co-localizes with aggregated proteins in neurodegenerative diseases (Chaplot et al., 2020), and Pcp4, a modulator of calcium signaling that is triplicated in DS (Mouton-Liger et al., 2011).