Gastric adenocarcinoma and esophageal adenocarcinoma are aggressive cancers with 
a poor prognosis. Therefore, new therapeutic strategies are needed, especially 
for patients refractory to conventional treatment. Cancer immunotherapy (CIT), 
is a promising new treatment option and is effective in a proportion of patients 
with gastroesophageal malignancies. Biomarkers for selecting patients likely to 
benefit from CIT in gastroesophageal malignancies remain unproven. Programmed 
cell death ligand-1 (PD-L1), which is a validated biomarker in non-small cell 
lung cancer (NSCLC), is often also used to select patients for CIT in the 
context of gastroesophageal cancer, although this marker has not been validated 
for this purpose. We question the use of PD-L1 as a biomarker in 
gastroesophageal cancers, as there are fundamental differences in PD-L1 
expression between NSCLC and gastroesophageal cancers. This review discusses the 
value of PD-L1 in selecting patients for CIT in esophageal and gastric cancer. 
Potential alternatives, especially microsatellite instability and Epstein-Barr 
virus positivity, are discussed.