The "long pentraxins" are an emerging family of genes that have conserved in their carboxy-terminal halves a pentraxin domain homologous to the prototypical acute phase protein pentraxins (C-reactive protein and serum amyloid P component) and acquired novel amino-terminal domains. In this report, a genomic fragment of 1371 nucleotides from the human "long pentraxin" gene PTX3 is characterized as a promoter on tumor necrosis factor-␣ (TNF␣) and interleukin (IL)-1 exposure in transfected 8387 human fibroblasts by chloramphenicol acetyltransferase and RNase protection assays. In the same cells, the PTX3 promoter does not respond to IL-6 stimulation. Furthermore, IL-1 and TNF␣ responsiveness is not seen in the Hep 3B hepatoma cell line. The minimal promoter contains one NF-B element which is shown to be necessary for induction and able to bind p50 homodimers and p65 heterodimers but not c-Rel. Mutants in this site lose the ability to bind NF-B proteins and to respond to TNF␣ and IL-1 in functional assays. Sp1-and AP-1 binding sites lying in proximity to the NF-B site do not seem to play a major role for cytokine responsiveness. Finally, cotransfection experiments with expression vectors validate that the natural promoter contains a functional NF-B site.The human gene hPTX3 has been recently cloned from interleukin-1 (IL-1b) 1 -stimulated endothelial cells (1) and from tumor necrosis factor-␣ (TNF␣)-stimulated fibroblasts (2). PTX3 belongs to the family of pentraxins (so named because they are assembled in pentamers) that include C-reactive protein (CRP) and serum amyloid P component (SAP) from several different species (3) and which are markers of the acute phase. Moreover, while the 3Ј half of PTX3 can be aligned with the full-length sequences of CRP and SAP (4) (pentraxin domain), the 5Ј half of the protein does not show significant homology with other known proteins.