Prostaglandin D2 (PGD2) is one of the five principal prostaglandins enzymatically derived from PGH2. It is abundantly generated in the cerebrospinal fluid (CSF) by lipocalin-type PGD synthase and in peripheral regions by myeloid cells, such as mast cells and basophils, via leukocyte-type PGD synthase. The compound 1,25-trans-PGD2 is an isomer of PGD2, characterized by the alteration of the double bond between carbons 5 and 6 from cis(Z) to trans(E). This trans isomer, found as a 2-5% impurity in most commercial PGD2 bulk drug preparations, is primarily synthesized as an analytical standard to identify and quantify this impurity. Based on existing studies of trans isomers of F-type prostaglandins, 5-trans-PGD2 likely exhibits biological activity comparable to its cis isomer, although no specific published reports confirm this for 5-trans-PGD2.