Muir-Torre syndrome (MTS; MIM 158320) is an autosomal dominant predisposition to skin tumours and various internal malignancies. Clinical criteria for a diagnosis of MTS are the synchronous or metachronous occurrence of at least one sebaceous gland neoplasia and at least one internal neoplasm in a patient (regardless of the family history).^1,2 The sebaceous gland neoplasias comprise adenomas, epitheliomas (sebaceomas), and carcinomas. In contrast, the frequent sebaceous gland hyperplasia is not indicative of MTS.^2,3 According to Schwartz and Torre,² the sebaceous neoplasias precede the internal neoplasias or are concurrent with them in 41% of MTS patients. As sebaceous gland neoplasias are rare, MTS should always be suspected when a sebaceous tumour has been diagnosed. Cystic sebaceous neoplasia is probably the most sensitive marker for this tumour predisposition syndrome.^2,4–6 Colorectal cancer is by far the most common internal malignancy in MTS patients.⁷ The spectrum of internal malignancies in MTS is similar to the various tumour entities observed in hereditary non-polyposis colorectal cancer (HNPCC; MIM 114500). HNPCC is an autosomal dominant cancer predisposition syndrome characterised by early onset of colorectal cancer and other associated tumours.^8,9 Several genes underlying HNPCC which are involved in DNA mismatch repair (MMR) have been identified within the last decade.^10–13 Germline mutations in the DNA MMR genes were detected in a high proportion of MTS patients, demonstrating that MTS most often represents a phenotypic variant of HNPCC.^14,15 Due to the underlying genetic mechanisms of tumourigenesis, tumours of these MTS patients exhibit high microsatellite instability (MSI-H), the characteristic feature of HNPCC tumours.¹⁶ Microsatellite analysis in tumour tissue of MTS patients therefore provides a useful tool to pre-select patients for mutation analysis in DNA MMR genes.³ Immunohistochemical testing for expression of the MSH2 and …