High levels of endogenous MGMT in tumor cells are believed to protect the tumor from alkylating agents used in chemotherapeutic regimen and MGMT levels may be an important parameter of treatment failure. Therefore, some investigators prefer the somewhat simpler immunohistochemical approach to detect the expression of the MGMT protein. Compared to MS-PCR, immunohis-tochemistry is a more reliable method if only FFPE tissue is available. However,BI-D1870 the relevance of MGMT-immunoreactivity is a matter of intense discussion especially when MGMT-immunoreactivity is correlated to MGMT promoter methylation status. We therefore evaluated 285 brain metastases for MGMT expression by immunohistochemistry. In about one third of the cases more than 95% of the tumor cells were MGMT-immunopositive, whereas in one third no immunoreactivity was detectable. The remaining cases showed a heterogeneous MGMT-immunoprofile ranging from 5 to 95%. In 178 cases, MS-PCR and immunohistochemical data was available. We found a strong correlation between homogeneous MGMT-immunoreactivity and unmethylated MGMT promoter. MGMT-immunoreactivity and evidence of promoter methylation in 9% of the samples may reflect differences in the methylation status of the MGMT promoter in tumor cell subpopulations as it is reported for malignant melanoma. Furthermore, extensive MGMT promoter methylation has been shown to go along with MGMT gene expression under certain conditions. A negative MGMT-immunostaining, however,Oligomycin A was not correlated with a defined promoter methylation status, possibly because methylation of the MGMT promoter is not necessarily linked to MGMT protein expression. Other mecha-nisms of gene silencing including gene deletion or mutation may lead to loss of protein expression-with or without promoter methylation. Moreover, MGMT is an inducible protein and lack of immunoreactivity at time of diagnosis might not reflect the potential functionality of the protein. MS-PCR proposes a clear MGMT promoter methylation status and divides the tumor samples into PCR-positive and –negative cases.