Selective cytotoxicity is an important goal of specific drug targeting. Toward this end, toxins isolated primarily from higher plants and bacteria have been coupled to monoclonal antibodies (MAbs) and evaluated for their clinical efficacy in cancer, AIDS, and immunological diseases (1,2). Immune responses against murine monoclonal antibodies MAbs (3,4) and antitoxin antibodies have been detected in both animals and humans treated with immunotoxins (ITs) (5-7) and present a major obstacle to the successful application of this technology. Although development of humanized antibodies have alleviated some of these effects (8, and references therein), the toxins themselves remain a problem. Consequently, the identification of human proteins to be used as components of immunoconjugates is highly desirable.