Human polymerase delta-interacting protein 1 (PDIP1) is a tumor necrosis factor a and interleukin 6 inducible protein that interacts directly with proliferating cell nuclear antigen (PCNA) and the small subunit (p50) of DNA polymerase d. PDIP1 binds PCNA and p50 simultaneously and stimulates polymerase d activity in vitro in the presence, but not the absence, of PCNA. It has been suggested that PDIP1 provides a link between cytokine activation and DNA replication in eukaryotes. Here these authors report the cloning of two rat genes homologous to human PDIP1, termed rat PDIP1 and rat tumor necrosis factor-induced protein 1 (TNFAIP1). The rat PDIP1 is mapped to chromosome 1q36 cM region, spans approximately 18.7 kb, and is organized into six exons. The rat TNFAIP1 gene is mapped to chromosome 10q25 cM, spans approximately 12.9 kb, and is composed of seven exons. The deduced proteins of rat PDIP1 and rat TNFAIP1 share 63.1% sequence identity with each other and are highly conserved in the majority of the middle portion of the two proteins, which encode a BTB/POZ domain at the N-terminus and a PCNA binding motif (QTKV-EFP) at the C-terminus, respectively. The BTB/POZ domain and the PCNA binding motif are highly conserved during the evolution. Both rat PDIP1 and rat TNFAIP1 were demonstrated to interact with PCNA via BIAcore, GST pull-down, and co-mmunoprecipitation assays. Like the human PDIP1, both rat PDIP1 and rat TNFAIP1 stimulate polymerase d activity in vitro in a PCNA-dependent way. J. Exp. Zool. 303A: 227-240, 2005.