Human Epididymis Protein 4 (HE4) is also known as WAP four-disulfide core domain protein 2 (WFDC2), Epididymal secretory protein E4, Major epididymis-specific protein E4, Putative protease inhibitor WAP5, is a member of the WFDC domain family. HE4 / WFDC2 is expressed in pulmonary epithelial cells, and was also found to be expressed in some ovarian cancers. The WFDC domain, or WAP Signature motif, contains eight cysteines forming four disulfide bonds at the core of the protein, and functions as a protease inhibitor in many family members. HE4 is also a small secretory protein, which may be involved in sperm maturation.