CD274 and mastocytosis: In addition to the difference in the PD-L1 detection by mastocytosis subtype in serum versus by IHC on tissue biopsies, there was a variability seen in the percentage of mast cells staining for PD-L1 within the same splenic tissue reported in Hatch et al. This variability of staining within the same tissue has implications for the use of PD-L1 as a potential prognostic or predictive biomarker.