We analyzed the expression level of genes involved in active DNA demethylation (TET1, TET2, TET3, TDG) as well as in ascorbic acid transport (SLC23A1 and SLC23A2) in leukocytes, and the level of 5-mC and its derivatives in leukocytes and urine samples from breast cancer patients qualified to the study. This evidence concerns the gene TET3 and breast cancer.