EPSTI1 (Epithelial-stromal interaction protein 1), also known as BRESI1, is a 318 amino acid protein that is highly expressed in spleen, placenta, small intestine, liver, kidney, thymus, testis and salivary gland. Existing as three isoforms that are formed due to alternative splicing events, EPSTI1 is weakly expressed in normal breast tissue, but is heavily up-regulated in breast carcinoma, suggesting an important role for EPSTI1 in tumor formation and/or progression. Additionally, EPSTI1 is found in blood cells from systemic lupus erythematosus (SLE)-afflicted patients, implicating EPSTI1 as a potential protein involved in SLE.