Herein, we describe the biophysical characterisation of complexes formed between copper (II) carbosilane dendrimers (Gn-[Cu]) and pro-apoptotic siRNA, their ability to internalise the siRNA into cancer cells, and the cytotoxicity in human breast cancer cell line (MCF-7) in which Mcl-1 and Bcl-2 are overexpressed [22,24]. The gene discussed is MCL1; the disease is breast cancer.