Abstract Alterations of DNA mismatch repair (MMR) genes are involved in carcinogenesis of sporadic and in- herited human cancers characterised by instability of DNA microsatellite sequences (MSI). MSI tumours are usually identified using molecular analysis. In the pres- ent investigation, hMLH1 and hMSH2 immunohisto- chemistry was tested in order to evaluate the utility of this method in predicting MMR deficiency. Colorectal (72), gastric (68), endometrial (44) and ovarian (17) car- cinomas were independently evaluated for familial histo- ry, histological type of tumour, MSI status and immuno- histochemical results. Loss of expression of either hMLH1 or hMSH2 was observed in 51 of 55 (92.8%) MSI tumours, while 145 of 146 microsatellite stable (MSS) tumours expressed both the hMLH1 and hMSH2 gene products. Independently of tumour site, an overall agreement between immunohistochemical and molecular results was observed in 15 hereditary non-polyposis co- lorectal cancer-related tumours. Among sporadic tu- mours, only 2 of 60 colorectal and 2 of 66 gastric carci- nomas, displaying MSI, expressed both hMLH1 and hMSH2 gene products. All 39 endometrial and 16 ovari- an tumours presented a concordant molecular and immu- nohistochemical profile. These data show that immuno- histochemistry is an accurate and rapid method to predict the presence of defective DNA MMR genes and to iden- tify both sporadic and familial MSI tumours. Keywords Immunohistochemistry · Microsatellite instability · hMLH1 and hMSH2 genes · Gastric and colorectal carcinomas · Endometrial and ovarian carcinomas Introduction Defects in the DNA mismatch repair (MMR) system are involved in carcinogenesis and tumour progression of sporadic and inherited human cancers [10, 26]. MMR deficiency leads to the accumulation of base–base mis- matches and short insertion/deletion mispairs, generated as a consequence of DNA replication errors and homolo- gous recombinations. Most cells deficient in MMR often display a high level of genomic instability (MSI-H), characterised by changes in repeat numbers of simple re- petitive sequences (microsatellite instability; MSI). In humans, MMR is mediated by at least five genes, includ- ing hMLH1, hMSH2, hMSH3, hMSH6 and hPMS2 [23]. Inherited mutations of hMLH1 and hMSH2 genes have been demonstrated as the cause of 70–100% of heredi- tary non-polyposis colorectal cancers (HNPCC), show- ing MSI-H [1, 3, 12, 28, 32]. On the contrary, germline hMSH3, hMSH6 and hPMS2 mutations have been rarely identified in HNPCC patients [2, 38]. MMR genes are also involved in the development of a subset of sporadic colorectal, gastric and endometrial tu- mours. In fact, MSI-H has been observed in 13–44% of gastric [17], 10–15% of colorectal [20, 31] and 17–23% of endometrial [4, 39] sporadic carcinomas. The MSI pheno- type, in these cases, is consistent with a somatic MMR de- fect. Recent studies pointed out that about 90% of sporad- ic MSI cancers have hMLH1 transcriptional silencing, while a minority of cases show inactivation of hMSH2 or hMLH1 due to somatic mutations [18, 45]. The evidence that epigenetic mechanisms cause inactivation of MMR genes [8] and, in addition, the identification of a high per- centage of missense variants of uncertain pathogenetic A.M. Chiaravalli · C. Facco · M.G. Tibiletti · C. Riva C. Capella ( ✉ ) Department of Pathology, Hospital Fondazione Macchi, viale Borri 57, I-21100 Varese, Italy e-mail: Carlo.Capella@ospedale.varese.it Tel.: +39-332-278231, Fax: +39-332-278599 D. Furlan · A. Dionigi · B. Casati · L. Albarello · C. Capella Department of Clinical and Biological Sciences, University of Insubria, Varese, Italy Virchows Arch (2001) 438:39–48 DOI 10.1007/s004280000325 ORIGINAL ARTICLE Anna Maria Chiaravalli · Daniela Furlan Carla Facco · Maria Grazia Tibiletti Adriana Dionigi · Barbara Casati Luca Albarello · Cristina Riva · Carlo Capella Immunohistochemical pattern of hMSH2/hMLH1 in familial and sporadic colorectal, gastric, endometrial and ovarian carcinomas with instability in microsatellite sequences Received: 2 June 2000 / Accepted: 11 September 2000 / Published online: 31 October 2000 © Springer-Verlag 2000