AIM:To clarify the correlation with phenotypic expression,clinicopathological features,genetic alteration and microsatellite-instability status in small intestinal adenocarcinoma(SIA).METHODS:The cases of 47 patients diagnosed with primary SIAs that were surgically resected at our institution in 1975-2005 were studied.We reviewed clinicopathological findings(age,gender,tumor size,gross appearance,histological morphologic type,invasion depth,lymphatic permeation,venous invasion,and lymph node metastasis),and the immunohistochemical expression of MUC5 AC,MUC6,MUC2,CD10,and mismatch-repair(MMR) proteins(MLH1 and MSH2).We analyzed KRAS and BRAF gene mutations,and the microsatellite instability(MSI) status.The immunohistochemical staining of CD10,MUC2,MUC5 AC and MUC6 was considered positive when distinct staining in > 5% of the adenocarcinoma cells was recorded.To evaluate of MMR protein expression,we used adjacent normal tissue including lymphoid follicles,inflammatory cells,and stromal cells as an internal positive control.Sections without nuclear staining in the tumor cells were considered to have lost the expression of the respective MMR protein.RESULTS:There were 29 males and 18 females patients(mean age 59.9 years,range:23-87 years).Tumors were located in the duodenum in 14 cases(30%),the jejunum in 21 cases(45%),and the ileumin 12 cases(25%).A phenotypic expression analysis revealed 20 MUC2-positive tumors(42.6%),11 MUC5AC-positive(23.4%),4 MUC6-positive(8.5%),and 7 CD10-positive(14.9%).The tumor sizes of the MUC2(+) tumors were significantly larger than those of the MUC2(-) tumors(mean,5.7 ± 1.4 cm vs 4.7 ± 2.1 cm,P < 0.05).All three tumors with adenomatous component were positive for MUC2(P < 0.05).Polypoid appearance was seen significantly more frequently in the CD10(+) group than in the CD10(-) group(P < 0.05).The tumor size was significantly larger in the CD10(+) group than in the CD10(-) group(mean,5.9 ± 1.4 cm vs 5.0 ± 2.1 cm,P < 0.05).Of 34 SIAs with successfully obtained MSI data,4 were MSI-high.Of the 4 SIAs positive for both MUC5 AC and MUC2,3 showed MSI-H(75%) and 3 were mucinous adenocarcinoma(75%).KRAS mutations were detected in 4 SIAs.SIAs had KRAS mutation expressed only MUC2,but were negative for MUC5 AC,MUC6 and CD10.CONCLUSION:These findings suggest that the phenotypic expression of SIAs is correlated with their biological behavior,genetic alteration,and MSI status. AIM: To clarify the correlation with phenotypic expression, clinicopathological features, genetic alterations and microsatellite-instability status in small intestinal adenocarcinoma (SIA). METHODS: The cases of 47 patients diagnosed with primary SIAs that were surgically resected at our institution in 1975-2005 were studied. We reviewed clinicopathological findings (age, gender, tumor size, gross appearance, histological morphologic type, invasion depth, lymphatic permeation, venous invasion, and lymph node metastasis), and the immunohistochemical expression of MUC5 AC, MUC6, MUC2, CD10 , and mismatch-repair (MMR) proteins (MLH1 and MSH2) .We analyzed KRAS and BRAF gene mutations, and the microsatellite instability (MSI) status.The immunohistochemical staining of CD10, MUC2, MUC5 AC and MUC6 was considered positive when distinct staining in> 5% of the adenocarcinoma cells was recorded. To evaluate of MMR protein expression, we used adjacent normal tissues including lymphoid follicles, inflammatory cells, and stromal c There are 29 males and 18 females patients (mean age 59.9 years, range: 23-87 years ) Tumors were located in the duodenum in 14 cases (30%), the jejunum in 21 cases (45%), and the ileumin 12 cases (25%). A phenotypic expression analysis revealed 20 MUC2-positive tumors , 11 MUC5AC-positive (23.4%), 4 MUC6-positive (8.5%), and 7 CD10-positive (14.9%). The tumor sizes of the MUC2 (+) tumors were significantly larger than those of the MUC2 tumors (mean, 5.7 ± 1.4 cm vs. 4.7 ± 2.1 cm, P <0.05) .All three tumors with adenomatous component were positive for MUC2 (P <0.05) .Polypoid appearance was seen significantly more frequently in the CD10 (+) group than in the CD10 (-) group (P <0.05). The tumor size was significantly larger in the CD10 (+) group than in CD10 (-) group (mean, 5.9 ± 1.4 cm vs 5.0 ± 2.1 cm, P <0.05 ) .Of 34 SIAs with succMSE-H (75%) and 3 were mucinous adenocarcinoma (75%). KRAS mutations were detected in 4 SIAs. SIAs had KRAS mutation expressed only MUC2, but were negative for MUC5 AC, MUC6 and CD10. CONCLUSION: These findings suggest that the phenotypic expression of SIAs is correlated with their biological behavior, genetic alteration, and MSI status.