Apoptosis is related to many diseases and development. The p53 tumor-suppressor protein induces apoptosis through transcriptional activation of several genes. A novel p53 inducible gene was identified recently and designated p53DINP1 (for p53-dependent damage-inducible nuclear protein 1) and SIP (for stress induced protein) in human and mouse. A p53DINP1 antisense oligonucleotide inhibits and overexpression of p53DINP1 enhances Ser46 phosphorylation of p53, induction of p53AIP1, and cell death induced by DNA double-strand breaks. p53DINP1 may regulate p53-dependent apoptosis through phosphorylation at Ser46 and induction of p53AIP1. The p53DINP1/SIP gene encodes two proteins of 27 and 18 kDa in human and mouse termed p53DINP1-alpha and p53DINP1-beta or SIP27 and SIP18. p53DINP1/SIP is expressed in many tissues and induced by a variety of stress agents including UV stress, mutagenic stress, heat shock, and oxidative stress.