NGF and endometriosis: Since endometriosis is associated with abdominal pain and nerve fiber outgrowth within the lesions, we further examined the CpG methylation changes in the BDNF gene, which codes for a secreted nerve growth factor that was shown to be elevated in tissue samples of patients with endometriosis vs. controls [71] and for which a loss of function polymorphism has been associated with endometriosis-related infertility [72].