The viability of human umbilical vein endothelial cells (HUVECs) at day 2 postirradiation to various X-ray doses. Immediately after irradiation, HUVECs, which were initially cultured in normal medium, were placed in hypoxanthine (Hx)-removed or added medium for 48 h. The WST-1 (A) and cell count methods (B) were used to assess differences in cell viability with and without Hx (n = 3 each). Gray bars represent cell viability in media containing endogenous Hx (2 µM), and white bars represent cell viability in media with endogenous Hx removed. The numbers under white bars indicate the final concentration of exogenous Hx added to media. For the WST-1 method, the value of the formazan product produced (450 nm) minus the value of the turbidity of the medium (630 nm) was plotted. Data are shown as mean ± SEM. *P < 0.05, **P < 0.01. Note that the scale of the Y-axis has been changed in the graph to get a clearer presentation.