CD99 and glioblastoma: As it has been widely reported that overexpression of CD99 could increase the migration and invasiveness of GBM cells (Seol et al., 2012; Cardoso et al., 2019), we deduced that although CD99 and other invasion-associated PCGs play key roles in regulating tumor cell invasion, their mediated mechanisms were distinct and redundant, resulting in their mutually exclusive expression patterns.