PRIMARY: In Phase I, to define a broadly tolerable dose of isotretinoin that can be used in combination with interferon alfa-2a (IFN alfa-2a). In Phase II, to determine trends in efficacy of isotretinoin alone or in combination with IFN alfa-2a as chemoprevention (preventing progression or recurrence) of anal intraepithelial neoplasia ( AIN ) / squamous intraepithelial lesions ( SIL ) in patients with HIV infection.