The Sundance Channel, a joint venture of NBC Universal, the CBS Corporation and the actor and director Robert Redford, is up for sale, a media analyst said Wednesday. Sundance, a 12-year-old cable channel currently accessible to 26 million homes, is seeking more than $400 million, according to the analyst, Richard Greenfield of Pali Research. A spokeswoman for the channel declined to comment in an e-mail message. If neither NBC nor CBS decide to take full ownership, Mr. Greenfield said, Cablevision, Time Warner and Viacom could be interested. Some cable customers receive Sundance as part of a digital channel package; others must subscribe separately. The Sundance Channel promotes itself as “independent-minded” and shows films, documentaries and original programming. It operates separately from the Sundance Film Festival but seeks a similar audience. Its programs include “Iconoclasts,” in which two famous people interview each other, and the environmentally oriented series “The Green.” Rather than running standard commercials, the channel has allowed marketers to sponsor specific programs.