Tober JM, et al. - Researchers examined the expression of ARID1A in whole tissue sections (WTS) of gastric cancer (GC) testing the following hypotheses: ARID1A shows intratumoral heterogeneity, and ARID1A expression and/or heterogeneity correlates with clinicopathological patient characteristics. They analyzed ARID1A expression by immunohistochemistry in 450 primary GCs and 143 corresponding lymph node metastases. This study compared expression patterns with clinicopathological characteristics and patient survival. They explored ARID1A genotype and CpG methylation status in 7 GCs with a heterogeneous “black-and-white” expression pattern. The evidence showed that ARID1A loss was heterogeneous in a subgroup of cases, which implies that ARID1A mutations might be a later event in gastric carcinogenesis.