We used GlaI/FatI-PCR assay for determination of 5mC/T polymorphism frequencies at position xp1: 245618129 (according to the GRCh38.p13 genomic assembly) in DNA preparations isolated from blood cells of 51 breast cancer (BC) patients and 63 gastric cancer patients (GC). The data obtained were compared with those previously obtained for 92 healthy blood donors. It was shown that the cytosine in this position is predominantly is methylated in all DNA samples. The incidence of a diploid C/C set in this position in patients with breast cancer and gastric cancer is 9.8% and 7.94%, respectively. This is significantly different from the incidence of C homozygotes in healthy women (26.09%) and healthy donors of both sexes (22.83%). Thus, C/C homozygotes at position xp1: 245618129 is an favorable sign indicating a lower risk (2.7-2.9 times) of two types of oncological diseases. At the same time, the incidence of C/T heterozygotes and T/T homozygotes in patients with gastric cancer and breast cancer exceeds similar indicators for control groups of healthy people by 5.5-10.7%.