Aneuploidy is frequently observed in cancers and is considered a crucial mechanism in cancer development. STAG2 is a gene that encodes a component of cohesion complex required for normal chromosomal segregation. Recently, somatic mutation of STAG2 gene and loss of STAG2 protein have been reported in glioblastoma, Ewing's sarcoma and melanoma. The aim of this study was to see whether such alterations of STAG2 are also common in other cancers. In this study, we analyzed STAG2 somatic mutation in 45 colorectal carcinomas (CRC), 45 gastric carcinomas (GC), 45 breast carcinomas, 45 non-small cell lung cancers and 45 prostate carcinomas (PCA) by single-strand conformation polymorphism. We analyzed also STAG2 protein expression in 100 GC, 103 CRC and 107 PCA by immunohistochemistry. STAG2 protein was well expressed in normal stomach, colon and prostate epithelial cells, while it was lost in 27% of GC, 23% of CRC and 30% of PCA. The loss of STAG2 was observed irrespective of subtypes, stages and grades of the cancers. However, we could not find any STAG2 mutations in these cancers. The loss of expression of STAG2 in GC, CRC and PCA tissues compared to their corresponding normal cells indicates that STAG2 loss is common in carcinomas as well. The data suggest also that loss of expression of STAG2, but not somatic mutation, might be responsible to STAG2 inactivation and is common in studied types of carcinomas.