If the levels of cytokeratin-19 fragments (CYFRA 21-1) were analyzed based on the phase of the menstrual cycle, the average levels of cytokeratin-19 fragments (CYFRA 21-1) obtained in the proliferative phase were significantly higher than the secretory phase in both endometriosis and non-endometriosis groups. The gene discussed is KRT19; the disease is endometriosis.