In a study of archived biospecimens, the density of nerve fibers was fourteen times higher in the endometrium of women with rAFS stage I-II endometriosis relative to that of healthy women, and the combination of PGP9.5, substance P, and vasoactive intestinal peptide was 95% sensitive and 100% specific for the diagnosis of endometriosis [141]. This evidence concerns the gene VIP and endometriosis.