Generic relationships play an important role in conceptual modeling. The most used generic relationships are association, specialisation/generalisation, classification/instantiation and aggregation/decomposition. Other generic relationships have been identified in the literature, such as role-of which represents the dynamic aspects of objects. These dynamic aspects can be correctly modeled neither by the generic relationships offered by UML nor by its own concept of roles involved in collaboration diagrams. To fill this gap, this work proposes an extension of UML by role-of. New metaclasses and OCL rules are added to the basic metamodel of UML to capture the semantics of roles.