GPC6 and neoplasm: Additional analyses of publicly available datasets confirmed GPC6 expression in tumor cells only (Supplementary Fig. 4), as well as reduced survival depending on GPC6 expression across different glioma subtypes (Supplementary Fig. 5) To assess changes in the neighboring synaptic microenvironment, we stained peritumoral sections for excitatory and inhibitory synapses in developing tumors at P30 (Fig. 2G, H) and found a significant increase in excitatory synapses but no significant change in inhibitory synapses in the presence of GPC6.