SLC6A4 and gestational diabetes: The SLC6A4 DNA methylation levels at 9 of 10 CpG sites were significantly higher in the GDM group than in the control group (Figure 1a), and the mean SLC6A4 DNA methylation was also higher in the GDM group (2.29 ± 0.06 vs. 1.16 ± 0.07%; p < 0.001; Figure 1b).