CD274 and meningioma: To assess PD-L1 expression in tissue resection specimens and to correlate expression with meningioma grade in our TMA cohorts, we first developed and tested a custom-designed RNAscope probe (see Materials and Methods) for detecting human PD-L1 mRNA transcripts by in situ hybridization in FFPE tissues with 3,3′-Diaminobenzidine (DAB) used for signal visualization.