HCN1 and breast cancer: The polymorphisms under study have been previously genotyped in the MEC, have all been identified in GWAS, and either were the most strongly associated with breast cancer (rs2981582 in FGFR2 [8], rs3803662 and rs12443621in TOX3 [8,9], rs889312 in MAP3K1 [8], rs13387042 at 2q [9]), have shown some indication of association with mammographic density in previous studies (rs3817198 in LSP1 [11,12]), or are close to a region found to be linked to mammographic density (rs981782 and rs10941679 near HCN1/MRPS30 [4]).