PGR and cancer: Interestingly, most of the FDA-approved protein biomarkers for human cancers are membrane proteins, including cancer antigen CA125 (ovarian), carcinoembryonic antigen (colon), epidermal growth factor receptor (colon), tyrosine-protein kinase KIT (gastrointestinal), HER2/NEU, CA15-3, CA27-29, Oestrogen receptor and progesterone receptor (breast) and bladder tumour-associated antigen (bladder) [31].