1a,1b-Dihomo Prostaglandin E2 (PGE2) is a rare polyunsaturated fatty acid first identified in extracts of sheep vesicular gland microsomes, known to contain COX , incubated with adrenic acid. 1a,1b-Dihomo PGE2 has also been identified in conditioned media of RAW 264.7 macrophages stimulated with endotoxin and arachidonic acid. This product is thought to be produced by elongation of AA to adrenic acid, which is then metabolized sequentially by COX and PGE synthase.