Human epididymis protein 4 (HE4) belongs to the whey acidic four-disulfide core protein family, and its biological function is currently unknown. HE4 expression occurs in the glandular epithelium of the reproductive tract, distal renal tubules, and respiratory epithelium. In cancer, HE4 levels can have utility as a prognostic biomarker for epithelial cancer, serous ovarian cancer, and endometrioid cancer. The primary established clinical application of HE4 is in post-therapy monitoring of ovarian cancer patients who had elevated pretreatment levels with little or no expression of CA125.