The primary objective of this study is to assess microsatellite instability status in tumor specimens of 15 HIV-positive patients from the Swiss Cohort who were diagnosed with SCCA; an interim analysis will then be performed to determine the incidence of MSI in SCCA of HIV-positive patients. If a majority of tumours exhibit MSI in this population, we will proceed with the second part of the study, which objective is to perform the same analyses in similar tumors of 15 HIV-negative patients, and thus to compare the molecular profiles of SCCA in both populations. By doing so, the potential role of immune suppression in carcinogenesis progression might be established.