Five ionomycin-inducible complementary cDNAs, designated PAR1, 2, 3, 4 and 5, have been isolated from the prostate cancer cell line AT-3. Nucleotide sequencing identified PAR1 as the rat homolog of monkeyP-1, PAR2 as the injury-inducible gene HB-EGF, and PAR3 as the serum-induced gene cyr-61. PAR4 and PAR5 sequences were not found to correspond to any previously described proteins. PAR4 (prostate apoptosis response-4) is specifically expressed by cells entering apoptosis and is not inducible by growth factor stimulation, oxidative stress and necrosis, or growth arrest. The PAR4 gene encodes a 38 kDa protein with a putative nuclear localization signal and carboxy terminal leucine zipper.