Due to the nature of observational studies,selection of endometriosis patients to either the GnRH-agonist orGnRH-antagonist protocol may be influenced by the clinician’s preferences.Fertility centres have also gained more experience with the GnRH-antagonistprotocol throughout the years and an updated RCT would be preferred tominimize selection bias. This evidence concerns the gene GNRH1 and endometriosis.