However, homozygous XPDXP (XPDR683W) and hemizygous XPDXPCS (XPDG602D) human cells are known to be highly sensitive to UV [19,25], as are cells from a homozygous viable XpdXPCS/XPCS (XPDG602D/G602D) mouse model (Figure 4A, dotted line) [23].