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Introduction: Multiple endocrine neoplasia type 1 syndrome (MEN 1) is caused by mutations in MEN1 gene, encoding menin protein. A combination of pituitary adenomas (PA) and primary hyperparathyroidism (PHPT) in patients without MEN1 mutations is defined as MEN 1 phenocopy. The aim of the study is to find if there are any differences in menin expression in PA of patients with genetically confirmed MEN 1, MEN 1 phenocopies and sporadic acromegaly. Materials and Methods: Formalin-fixed paraffin-embedded PA tissues were obtained from 9 genetically confirmed MEN 1 patients (group 1), 12 patients with MEN 1 phenocopies (a combination of PA and PHPT) (group 2) and 14 patients with sporadic acromegaly (group 3). The integrity of the tissues was tested by immunohistochemistry (IHC) using antibodies against the nuclear protein Pit-1. MEN1 mutations were confirmed or excluded by Sanger sequencing or by NGS (NextSeq550, Illumina, USA). Expression of menin was assessed using IHC (Anti-Menin antibody — ChIP Grade, Abcam, UK). Results: The distribution of PA by the type of secretion in group 1 was: 3 — ACTH-secreting, 2 — PRL-secreting, 2 — GH+PRL, 1 — TSH+PRL, 1 — ACTH+PRL, 1 — silent gonadotroph adenoma. All 12 PA from group 2 were GH-secreting. All 14 PA in group 3 were GH-secreting without mixed secretion. The median age at the moment of transsphenoidal surgery in group 1 was 35 years [27; 47], in group 2 — 59 years [56; 65], in group 3 — 56 years [53; 62]. There were 2 men and 7 women in group 1; 2 men and 10 women in group 2; 4 men and 10 women in group 3. In group 1 patients did not receive somatostation analogues (SSA), 7 patients received cabergoline. In group 2 seven patients received SSA, in group 3 — 4 patients received SSA, 2 patients received cabergoline. The results of menin staining were (negative staining/positive cytoplasmic staining/positive nuclear staining): group 1 — 4/4/0; group 2 — 0/11/1; group 3 — 1/7/6. Phenocopy group showed significantly more cytoplasmic expression of menin than in MEN 1 group (p Conclusion: Menin expression is generally preserved in PA in MEN 1 phenocopies and sporadic acromegaly, though with different pattern of nuclear and cytoplasmic expression, while its expression varies in PA in MEN 1. These findings suggest that pathogenesis of PA in MEN 1 phenocopies and sporadic acromegaly may have similarities. The mechanisms of co-occurrence of PA and PHPT in MEN 1 phenocopies are poorly understood and epigenetic modifications downstream menin interacting pathways could play a role.