The present invention relates to: pharmaceutical compositions for inducing apoptosis in cells, the pharmaceutical compositions comprising a prostacyclin synthase gene as an active ingredient; pharmaceutical compositions for gene therapy of cancer, the pharmaceutical compositions comprising the above pharmaceutical compositions for inducing apoptosis as an active ingredient; and a screening method for an agent that induces apoptosis in a cell, the method comprising determining the activation of peroxisome proliferator-activated receptor (PPAR)-.delta. in the presence of a test substance. The pharmaceutical composition of the present invention for inducing apoptosis enables treatment of diseases on which induction of apoptosis has a therapeutic effect. In addition, the pharmaceutical composition of the present invention for gene therapy can lead cancer cells to cell death. Furthermore, the screening method of the present invention can be used conveniently to screen for agents capable of inducing apoptosis.