Abstract Thyroid carcinoma is the most prevalent endocrine neoplasm. The most recurrent mutation in thyroid cancer (TC) is the V600E substitution in BRAF, which induces the over-activation of the MAPK pathway. The interaction between this pathway and the deregulation of genes that facilitate tumorigenesis such as CLDN1 could be useful for decision-making related to surgical removal and the degree of initial resection; however, there is a lack of information on the association of CLDN1 expression with BRAF V600E. Objective. Evaluate the gene expression of CLDN1 in BRAF-mutated and BRAF-unmutated thyroid nodules. Methods. RT-qPCR was performed to assess BRAF genotyping (rs113488022) and CLDN1 expression in 94 thyroid nodule biopsy samples (TC=48). Clinicopathological data was retrieved to establish further associations. Results. Mutated BRAF thyroid nodules were found to be 7.14 ± 0.293 times more expressed than those without the mutation (p &lt 0.001, N=94). BRAF V600E mutation was identified in patients with TC exclusively. CLDN1 expression was 41.315 ± 0.554 times greater in patients with TC than in those without TC (p &lt 0.001). Heterozygous patients had a change factor of 6.8, whereas mutated homozygotes had a change factor of 7.5 (p &lt 0.001, N=94). CLDN1 mean expression is significantly higher in A allele carriers compared to normal homozygotes (p &lt 0.001). When analyzing just within the TC group, the behavior is maintained, suggesting a potential association between the mutated allele and the expression of these genes. Conclusion. CLDN1 is significantly upregulated in TC patients carrying the BRAF V600E mutation. BRAF V600E is only observed in patients with TC, implying a biological connection between them, which can be explained via MAPK; subsequent studies are needed to confirm the correlations. Citation Format: Andrea Ross Orozco, Eliakym Arámbula Meraz, Anette Roxana Gastelum Quiroz, Fred Luque Ortega, Edith Eunice García Alvarez, Marco Álvarez Arrazola, Karla Lizbeth Morales Hernández, Noemí García Magallanes. BRAF V600E is associated with the upregulation of CLDN1 in thyroid cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB050.