PDCD1 and glioblastoma: Additionally, KYNU expression positively correlated with the expression of immunoregulatory cytokines (IL-10 and TGF-β) and immune checkpoints inhibitors, such as PD-1 and PDL-1 and CTLA-4, as well as with matrix metalloproteinase 9 (MMP-9), an enzyme involved in the breakdown of the extracellular matrix that promotes tumoral invasion in GBM and is correlated with poor prognosis [69].