Copper homeostasis in human cells. Cu enters via hCTR1 and is handed off to copper chaperones, ATOX1, CCS, and COX17, which then deliver Cu to its target proteins. MT, metallothionein may serve as a storage site. The efflux proteins, ATP7A and ATP7B, reside in the trans-Golgi and secretory pathway and relocate via vesicle trafficking and fusion, in conditions of high intracellular Cu, to the plasma membrane.