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Cancer biologists are constantly hunting for tumor-specific antigens that can be targeted for monoclonal antibody therapy. Work by Andrea Schietinger, Mary Philip, Hans Schreiber, and colleagues (University of Chicago, IL) reveals that you don't need a mutation in the antigen itself to make it tumor specific. Incorrect posttranslational modification will do.

The group identified a mutation-free antigen in a spontaneous murine fibrosarcoma, called Ag104A. Mice with this tumor produce a high-affinity, highly specific antibody against what the group now identifies as a transmembrane protein called OTS8. Although OTS8 is widely expressed, the antibody was only reactive to OTS8 from the Ag104A tumor.

Tumor-derived OTS8 was a different molecular weight than its nonantigenic counterpart yet, to the group's surprise, was not mutated. The different molecular weight was instead due to...