Gross cystic disease is a common premenopausal disorder in which gross cysts are the predominant pathologic lesion. It is characterized by production of a fluid secretion which accumulates in the breast cysts. Gross cystic disease fluid is a pathologic secretion from breast composed of several glycoproteins, including a unique 15 kDa monomer protein, GCDFP 15. The cells within the body that produce GCDFP 15 appear to be restricted primarily to those with apocrine function. Studies have found GCDFP 15 to be a highly specific and sensitive marker for breast cancer.