All commonly used cloning vectors in molecular biology have key features necessary for their function, such as a suitable cloning site and selectable marker.  Others may have additional features specific to their use.  For reason of ease and convenience, cloning is often performed using E. coli. Thus, the cloning vectors used often have elements necessary for their propagation and maintenance in E. coli, such as a functional origin of replication (ori).  The ColE1 origin of replication is found in many plasmids.  Some vectors also include elements that allow them to be maintained in another organism in addition to E. coli, and these vectors are called shuttle vector.