Human PgR exists in two forms: 116kDa (Bform) and 81kDa (A-form). It acts as ligand activated transcription factor to regulate expression of the target genes. Null mutation in PgR gene leads to pleiotrophic reproductive abnormalities. Expression of PgR has been suggested to reflect an intact estrogen regulatory machinery and therefore, predict better clinical response to endocrine therapy than ER alone. Ab-8 is occasionally expressed on nuclei of melanoma tissues.