Human peptidylarginine deiminase type IV (PAD4), a Ca²⁺-dependent enzyme known to convert arginine residues to citrulline residues in histones, has been shown to be associated with the development of rheumatoid arthritis. Recently, it was noted that the human PAD4 gene (PADI4) regulates the expression of estrogen-responsive genes by modifying the methylated arginine sites in histones H3 and H4. In this study, we demonstrated that PADI4 was expressed in MCF-7 cells and was responsive to estrogen at the transcriptional level. Using the luciferase reporter gene fused to wild type or mutated 5'-flanking region of PADI4, we characterized that as few as 348 bp upstream from the transcription initiation site were sufficient to direct transcription of the reporter gene. Chromatin immunoprecipitation and small interfering RNA assays revealed that AP-1, Sp1/Sp3 and NF-Y were cis-acting factors bound to the minimal promoter of PADI4 and that they regulated gene expression in a cooperative manner. Moreover, it was indicated that estrogen stimulated PADI4 expression through binding of ER to the upstream of the PADI4 gene and ER-mediated enhancement of AP-1, Sp1 and NF-Y levels. These findings indicated that estrogen stimulated PADI4 expression through both of the classical and non-classical estrogen receptor-mediated pathway.