Rabbit anti-human TOB (Transducer Molecule of erbB-2) reacts with a 45 kDa protein originally identified as a molecule that interacts with the ErbB-2 protein, a receptor protein tyrosine kinase (RPTK). TOB does not contain either SH2 or SH3 domains but is homologous with the N terminus of the anti-proliferative BTG-1 protein. Like BTG-1, TOB when exogenously expressed, is able to suppress the growth of NIH3T3 cells. This growth suppression is hindered by the presence of kinase-active p185erbB2.