Tumour resistance to cisplatin and carboplatin mediated by inadequate levels of platinum reaching target DNA. Platinum might enter cells using either transporters — a significant one being the copper transporter CTR1 — or by passive diffusion. Loss of CTR1 results in less platinum entering cells and, consequently, drug resistance. Once inside cells, cisplatin is activated by the addition of water molecules to form a chemically reactive aqua species. This is facilitated by the relatively low chloride concentrations that are found within cells. In the cytoplasm, the activated aqua species preferentially reacts with species containing high sulphur levels by virtue of their containing many cysteine or methionine amino acids. These species include the tripeptide glutathione or metallothioneins. In some platinum-resistant cancer cells, glutathione and metallothionein levels are relatively high, so activated platinum is effectively 'mopped up' in the cytoplasm before DNA binding can occur, thereby causing resistance. Finally, active export of platinum from the cells through the copper exporters ATP7A and ATP7B as well as through the glutathione S-conjugate export GS-X pump (also known as MRP2 or ABCC2) can contribute to platinum drug resistance. GSTs, glutathione S-transferases.