SKA2 and substance abuse: Based on recent publications that have provided evidence that both PTSD and substance abuse may confound SKA2 methylation [9, 21], we selected a subset of the GTP sample with no history of PTSD or drug use (N = 115; 6 cases, 109 controls), where a combination of SKA2 and the interaction biosignature predicted SI with an AUC of 0.73 (95 % CI 0.59–0.87; P = 0.050) (Fig. 1d; Table 4) [12].