HPRT1 and melanoma: To assess the possibility that DTIC was transformed into 2-AzaHX, we analysed the UV absorption spectrum of DTIC and observed that light exposure of DTIC led to the formation of a metabolite with maximal UV absorption at 256-295 nm, similar to that of 2-AzaHX (Additional file 4: Figure S2).Next we treated the melanoma cell lines with DTIC in the presence of HX, which should compete with 2-AzaHX for HGPRT binding and prevent its inhibitory effect.