p57 is a human tumor suppressor gene encoded on the 11p15.5 chromosomal sequence.  It achieves negative regulation of the cell cycle by blocking the G1/S phase transition in the cell cycle, thereby preventing cell proliferation and tumor formation. This gene is expressed by the maternal allele, and the expression of the paternal allele is suppressed by imprinting. In normal placenta, cytotrophoblast, intermediate trophoblast and decidual cells, and stromal cells are labeled with this antibody. Intervillous trophoblastic islands demonstrate nuclear labeling in all entities and serve as an internal control. Complete hydatidiform mole is produced from paternal DNA and do not express P57; some hybrid mole are triploid, consisting of one maternally and two paternally derived haploid genes, and express P57. Anti-p57 has been used as an aid in discriminating complete hydatidiform mole (CHM) (no nuclear labeling of cytotrophoblasts or stromal cells) from partial hydatidiform mole (PHM) (nuclear staining of both cytotrophoblasts and stromal cells) and hydropic abortion. ^1-6