The object of the present invention is to provide a protein useful for searching an apoptosis-inhibiting substance or an apoptosis-promoting substance, a gene encoding the above protein, a vector comprising the above gene, and a transformant comprising the above vector. The present invention provides an apoptosis-inducing protein of any one of the following (a), (b), and (c): (a) a protein having the amino acid sequence shown in SEQ ID NO: 2 of the sequence listing; (b) a protein, which has an amino acid sequence comprising a deletion, substitution, and/or addition of one or several amino acids with respect to the amino acid sequence shown in SEQ ID NO: 2 of the sequence listing, and has activity of inducing apoptosis; and (c) a protein, which has an amino acid sequence showing homology of 95% or more with the amino acid sequence shown in SEQ ID NO: 2 of the sequence listing, and has activity of inducing apoptosis.