In addition, given that caffeine, NADP+, and NAD+ all show a similar purine structure, and that the NADP+- or NAD+-dependent reactions catalyzed by metabolic enzymes are ubiquitous in living organisms, we further speculate that caffeine might also affect other NADP+- or NAD+-dependent metabolic enzymes such as the NAD+-dependent and rate-limiting glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which is also known to play key roles in cancer development and progression (Yun et al., 2015; Liberti et al., 2017). This evidence concerns the gene GAPDH and cancer.