ID1 and endometriosis: Regarding the results from quantitative comparison of ID1, ID2, ID3, and ID4 gene expression between the control and endometriosis groups in the proliferative phase, the ID2 and ID3 gene expression in both ectopic and eutopic tissues were significantly higher than the control group and the expression level of ID1 and ID4 genes was decreased in the ectopic and also eutopic tissue.