Presumably the fifth subtype discovered, and usually separated into PGE1 and PGE2. These two, if made by the kidneys, slow sodium reabsorption; if within in the uterus, induce a stronger response to less stimulus; if made in the stomach lining inhibit gastric secretion; if secreted by macrophages, target tissues become more accessible to infiltration...and inflammation. These are the two prostaglandins whose levels are meant to be stabilised by gamma-linolenic acid (GLA) supplements. See PROSTAGLANDIN