The retinoblastoma (Rb) tumor suppressor is a well-established substrate for UL97 (1, 2, 4), whose phosphorylation can be detected both by the decreased electrophoretic mobility of the protein on SDS-PAGE, as well as by immunoblotting with a phosphospecific antibody (e.g., Rb-P-Thr-826). Here, RB1 is linked to neoplasm.