The molecular mechanisms involved in squamous cell carcinoma of the anus (SCCA) are poorly elucidated. HIV-positive and renal transplant patients are at high risk for developing SCCA, indicating that immune suppression plays a facilitating role. The investigators previously demonstrated that chromosomal instability (CIN) was more prevalent in SCCA of HIV-negative than HIV-positive patients. Hence, the investigators postulate that microsatellite instability (MSI), another molecular pathway, might be a feature of SCCA progression in the HIV-positive population.