COL4A1 and polycystic ovary syndrome: Key findings in the PCOS-endometrium include downregulation of collagen ligands COL1A1 in uNK2 and COL1A2 in proliferative stroma, fibroblasts and uNK2, whereas COL4A1, COL4A2 and COL6A3 are upregulated in AR+ epithelium, stroma 2 and SMCs (Fig. 5a and Supplementary Table 4a–e).