Copper(I)-NHC [CuCl(SIMes)] shows high cytotoxicity against human cancer cells. An increase of up to 150 fold compared with cisplatin was observed. The complex causes arrest of the cell cycle progression at the G1 phase concomitantly with apoptosis induction at low concentration (see scheme). This copper(I)-NHC also produces DNA strand breaks, which demonstrates its value as a Fenton-like reagent.