Patients from the Swiss HIV cohort with a biopsy-proven diagnosis of squamous cell carcinoma of the anus will be identified using the existing database. We choose to analyse at least 15 samples, depending on availability, between 3 SHCS centers. Tumors from HIV-negative patients who underwent surgery for SCCA will be retrieved from the Department of Pathology of University Hospital Geneva, pending review of the protocol by the local Ethics Committee.