Gross cystic disease of the breast is benign premenopausal disorder in which cysts are a predominant pathological lesion. These cysts appear to be formed from excessive apocrine cystic secretions. This fluid is composed of several glycoproteins including a unique 15kDa monomer protein, Gross Cystic Disease Fluid Protein-15 (GCDFP15). Cytosolic analysis of normal tissue specimens from all major organs has demonstrated GCDFP15 in apocrine epithelia, lacrimal, ceruminous and Moll’s glands and in numerous serous cells of the submandibular, tracheal, bronchial, sublingual and minor salivary glands. GCDFP15 and prostate specific antigen are co-expressed in androgen receptor-positive breast tumours.