TKTL1 and cancer: For this purpose, we incubated cancer cells in 1% oxygen, a growth condition that mimics that of most solid tumor microenvironment (Cairns et al., 2011), and observed a significant decrease in the levels of G6PD, TKTL1, and IDH1 in both Hela and MCF7 after WRN knockdown (Fig. S8, Supporting Information) that was comparable to that of cells grown under atmospheric oxygen.