Fig. 1

Flowchart of candidate CpG site screening. Initial screening was performed using the DNA methylation dataset of CRC in TCGA. The screening criteria were as follows: the methylation difference between CRC and normal tissues was Δβ ≥ 0.2 and was statistically significant (P < 0.01), and the methylation value of CpG sites in CRC tissues was β ≥ 0.2. Initially, 19,862 CpG sites with differences were screened. Next, the peripheral blood DNA methylation dataset of the non-tumor population in the GEO database was used for rescreening. The methylation levels of CpG sites in CRC tissues were compared with those in blood, and CpG sites with methylation values less than 0.25 were excluded. CpG sites (15,404) were identified. They were also ranked according to differences in their methylation values. Based on the comparison of some of the CRC-related methylation genes, it was found that the CpG sites of SDC2 and SHOX2 genes contained more and ranked higher among the 15,404 methylation sites, with a total of 17 CpG sites. Using the random forest algorithm and selecting CpG sites with low methylation values in normal tissues, the final two candidate CpG sites were obtained. CRC colorectal cancer