The present invention provides a signal transducer specifically expressed in mouse mast cells that has the amino acid sequence of SEQ ID No. 2, a signal transducer specifically expressed in human mast cells that has the amino acid sequence of SEQ ID No. 4, polynucleotides encoding these proteins, an expression vector involving these polynucleotides, transformed cells induced by these expression vectors, and antibodies against the foregoing proteins. The signal transducer provided in the present invention is useful for screening of novel medicines against allergic diseases.