IKIP (I kappa B kinase interacting protein)is located on chromosome 12 in close proximity to APAF1 (apoptotic protease-activating factor-1). IKIP was originally cloned from a yeast two hybrid screening as an IKK2 interacting protein. IKIP and APAF1 share a common 488 bp promoter from which the two genes are transcribed in opposite directions, presumably in a co-regulated manner. Three IKIP transcripts are generated by differential splicing and alternative exon usage and show no significant homology to other genes. IKIP is expressed predominantly in the lung, kidney, spleen, thymus and skeletal muscle. Like APAF1, expression of IKIP is enhanced by ?-irradiation, in a p53-dependent manner. When transfected into endothelial cells IKIP promotes apoptosis suggesting that it plays a role in the cell death process as well as NF-?B regulation.