P4 responsiveness may also be impaired by PR-coregulators and/or their downstream effectors, e.g. FKBP52, HIC-5/ARA55, KLF9/13, HOXA10, HAND2 or FOXO1, that play critical roles in PR-mediated endometrial functions, but are aberrantly expressed in eutopic endometrium and ectopic lesions of endometriosis [21, 23, 52, 53]. This evidence concerns the gene FKBP4 and endometriosis.