PR, a protein with 946 amino acids, is a ligand-activated transcription factor member of the steroid receptor super family of nuclear receptors. The functional structure is similar to that of estrogen receptor (ER), with considerable sequence homology in the DNA-binding central domain. PR is predominantly expressed in tumours of female sex steroid responsive tissues such as the mammary gland, endometrium and the ovary. About half of the breast carcinomas are ER+/PR+. A small fraction (<5%) is ER-/PR+. About half of the non-mucinous ovarian carcinomas are also PR+. From other PR-expressing tumours, meningiomas, various pancreatic neoplasms such as solid-pseudopapillary tumour and endocrine tumours, and salivary gland neoplasms are worth mentioning. The ER and PR status has been used for over 20 years as a predictor of breast carcinoma responsiveness to endocrine therapy and as a prognostic indicator for early recurrence. Up to 75% of ER+/PR+ breast carcinomas respond positively to endocrine treatment. ER+/PR- tumours are less responsive, and thus PR status adds information to ER-status. In combination the two predict benefit from endocrine therapy both in adjuvant setting and in advanced disease. In breast cancer predominance of one isoform, namely PR-B, is common. The majority of endometrial carcinomas express only one isoform. The applications of antibodies to PR are similar to those against ER, i.e. diagnosis of PR-positive tumours (often metastasis) and prediction of therapeutic response of breast carcinoma.