A cDNA for a novel protein termed Tob (Transducer of ErbB-2) that interacts with the c-erbB-2 gene product p185erbB2 was molecularly cloned. Nucleotide sequencing reveals that the Tob protein is a 45 kDa protein that does not contain either SH2 or SH3 domain but is homologous to the previously characterized anti-proliferative gene product BTG-1 at its amino-terminal half.