Endocervical gastric-type adenocarcinoma (GAS) is an aggressive type of endocervical mucinous adenocarcinoma characterized as being unrelated to human papillomavirus (HPV) and resistant to chemo/radiotherapy. In this study, we investigated the histology, immunohistochemistry patterns and molecular characteristics in a large cohort of GAS (n=62). Histologically, the majority of GAS cases exhibited a distinct morphology resembling gastric glands, although two exceptional cases exhibited HPV-associated adenocarcinoma (HPVA) morphology, while retaining the characteristic histology of GAS at the invasive front. By immunohistochemistry, Claudin18.2 emerged as a highly sensitive and specific marker for GAS. Additionally, the strong expression of Claudin18.2 in GAS patients indicated the potential of anti-Claudin18.2 therapy in the treatment of GAS. Other immunohistochemistry markers, including Muc6, p16, p53, Pax8, ER and PR, may provide additional diagnostic clues for GAS. Quantitative methylation analysis revealed that the overexpression of Claudin18.2 in GAS was governed by the hypomethylation of the CLDN18.2 promoter CpG islands. To further elucidate the pathogenic mechanisms of GAS and its relationship with gastric adenocarcinoma, we performed whole exome sequencing (WES) on 11 GAS and 9 gastric adenocarcinomas. TP53, CDKN2A, STK11 and TTN emerged as the most frequently mutated genes in GAS. Mutations in these genes primarily affected cell growth, cell cycle regulation, senescence and apoptosis. Intriguingly, these top mutated genes in GAS were also commonly mutated in gastric and pancreaticobiliary adenocarcinomas. Regarding germline variants, we identified a probably pathogenic variant in SPINK1, a gene linked to hereditary pancreatic cancer syndrome, in one GAS sample. This finding suggests a potential pathogenic link between pancreatic cancers and GAS. Overall, GAS exhibits molecular characteristics that resemble those observed in gastric and pancreaticobiliary adenocarcinomas, thereby lending support to the aggressive nature of GAS compared to HPVA.Copyright © 2024. Published by Elsevier Inc.