The present invention provides an genetically engineered cDNA of the rat bcl-x gene, which has at least one substitution selected from the substitutions that change residues 22 Tyr to Phe, residues 26 Gln to Asn and residues 165 Arg to Lys, in the coding region of rat bcl-x cDNA of SEQ ID NO: 1, a recombinant vector containing the engineered cDNA, a cell into which the recombinant vector was introduced, and an improved protein of Bcl-x.sub.L. The improved protein of Bcl-x.sub.L is useful as an ingredient for remedies for various diseases accompanied with cell death since it effectively inhibit cell death such as apoptosis.