SPP1 and polycystic ovary syndrome: We found no significant differences in the methylation levels of CpG sites or clusters of ANGPTL4, CYP17A1, PEX3, DIRAS3, PTX3, and SLC12A8 and amplicon for SPP1 (primer 1) in the GCs between PCOS and control women (Additional file 4: Figure S1a–h).