Gross cystic disease fluid protein (GCDFP-15), also called prolactin inducible protein (PIP), is a single polypeptide chain with a versatile function in human reproductive and immunological systems. GCDFP-15 binds to CD4, exerts a potent inhibition on T lymphocyte apoptosis mediated by CD4/T-cell receptor (TCR) activation, and carries a fibronectin-specific aspartyl protease activity. It is up regulated by prolactin and androgens, while it is down regulated by estrogen. In normal adult tissues, GCDFP-15 expression was found in all apocrine, lacrimal, ceruminous, and Moll's glands and in numerous serous cells of the submandibular, sublingual, and minor salivary glands. The serous cells of nasal and bronchial glands were also positive. It is used as a marker of apocrine differentiation. GCDFP-15 has been found in the cyst fluid of cystic breast disease and primary and metastatic breast cancer, and considered a highly specific marker for identification of breast cancer. GCDFP-15 expression has also been found in other cancer types including salivary glands, sweat glands, prostate, and lung.