Tober JM, et al. - ARID1A expression was investigated by immunohistochemistry in 450 primary gastric cancer (GC) and 143 corresponding lymph node metastases in this study of a large independent Central European cohort in whole tissue sections of GC in order to assess the the hypotheses that ARID1A exhibits intratumoral heterogeneity and ARID1A expression and/or heterogeneity relates to clinicopathological patient features. It was discovered that in 14.7% of GCs, heterogeneous or complete loss of ARID1A expression transpired and it related to PD-L1 status implying possibility for prospective combined anti-PD-L1/ARID1A therapy. Moreover, in a subgroup of cases ARID1A loss was heterogeneous which proposes that ARID1A mutations could be a delayed event in gastric carcinogenesis.