Breast cancer is the most frequently diagnosed cancer in women, with over 1 million new cases in the world each year. Recently, in addition to genetic mutations, numerous studies have found that epigenetics plays a direct role in the etiology of breast cancer. The PADIs are a family of epigenetic enzymes that catalyze citrullination, with previous work in our lab showing that PADIs can convert both protein and histone arginine to citrulline, leading to the disruption of protein-protein interactions, as well as direct transcriptional downregulation. Previous research has suggested a potential oncogenic role for PADI2 in breast cancer, though no formal analysis existed. The studies herein investigate the potential role of PADI2 as a novel oncogene and therapeutic target in the treatment of breast cancer in vitro and in vivo. First, using an in vitro model of breast cancer progression (MCF10AT), we show that PADI2 is upregulated upon the malignant transformation of cells, especially in MCF10DCIS cells, which recapitulate the highly invasive comedo-like ductal carcinoma in situ (DCIS) tumors seen in humans. Secondly, using RNA-seq, we show that PADI2 is highly correlated with HER2/ERBB2 overexpression across 57 breast cancer cell lines. We concluded this study by validating the use of our first-generation PADI inhibitor, Cl-amidine, as a therapeutic agent for the treatment of breast cancer both in vitro and in vivo. Following this, we further investigated the functional relationship between PADI2 and HER2 expression. Interestingly, PADI2 appears to function both upstream and downstream of HER2, potentially indicating a role in an oncogenic positivefeedback loop with HER2. Previous evidence from our lab established that PADI2 functions as an ER co-activator via the citrullination of histone H3 arginine 26 (H3R26) at ER-target gene promoters. We show here that PADI2 can bind to the HER2 promoter and downstream ERE; thus, suggesting that the epigenetic regulation of HER2 gene expression by PADI2 occurs via similar mechanisms to ER-target genes. Moreover, we were able to validate our highly potent next-generation PADI inhibitor, BB-Cl-amidine, in the treatment of breast cancer cells in vitro. Lastly, using a mouse model of PADI2 overexpression (MMTV-FLAGPADI2), we found that 20% of mice developed skin lesions after five months. These tumors express high levels of transgenic human PADI2 and display markers of increased inflammation and invasiveness-EMT. Furthermore, a subset of these tumors showed via histopathological analysis to have undergone malignant progression to highly invasive squamous cell carcinomas. Collectively, these studies provide functional and mechanistic evidence establishing PADI2 as a potential novel oncogene and target for cancer therapy.