Proline-rich acidic protein 1 (PRAP1) was originally isolated from mouse uterus in 1997. It is abundantly expressed in the uterus during late pregnancy but disappears from the uterus within 3 days after birth. It was therefore termed pregnant-specific uterus protein (PSUP). This protein is proline rich and acidic, thus it is also called proline-rich acidic protein 1 (PRAP1). Human genes of PRAP1 are located on chromosome 10q26.33. Proline-rich acidic protein 1 (PRAP1) is a secretory protein of 149 amino acids, with a 20 amino acids length signal peptide at its N-terminal. Human PRAP1 is abundantly expressed in the epithelial cells of the human liver, kidney, gastrointestinal tract and cervix. PRAP1 expression was found to be significantly down-regulated in hepatocellular carcinoma and right colon adenocarcinoma compared with the respective adjacent normal tissues. The function of PRAP1 is largely unknown. It is presumed that PRAP1 may play an important role in maintaining normal growth homeostasis in epithelia since PRAP1 protein was found to be over-expressed in apoptotic cells.  PRAP1 protein, has a domain similar to EF hand which function as a calcium binding site. Other proline-rich acidic proteins such as salivary proline-rich acidic proteins could bind calcium to maintain the concentration of ionic calcium and prevent bacterial adhesion. Small proline-rich proteins in the stratified squamous epithelium form a unique protective barrier against environmental insults. Proinflammatory cytokines are inducers of small proline-rich protein 1B expression in squamous metaplasia. Therefore, PRAP1 may function in modulating cell proliferation and apoptosis in the uterus and regulating immune protection for the embryo within the uterus. Overexpression of PRAP1 in cancer cells can inhibit cell growth. It has been suggested that PRAP1 may be involved in normal epithelial cell homeostasis as a negative regulator and that the down-regulation of PRAP1 in cancer cells may lead to dysregulated cell growth. The function and regulation of PRAP1 in the uterus remain unknown. Microarray analysis indicates that PRAP1 is down-regulated in the preimplantation day 3.5 uterus, but is greatly upregulated after implantation.