The paper presents an approach to semantic mutation of state machines that specify class behavior in Model-Driven Software Development. The mutations are aimed at different variants of UML state machine behavior. Mutation testing of a target application allows to compare different semantic interpretations and verify a set of test cases. We present a notation of a process combining model-driven development with semantic mutation and semantic consequence-oriented mutations. Origin and details of the proposed mutation operators are discussed. The approach has been supported by the Framework for eXecutable UML (FXU) that creates a C# application from UML classes and state machines. The tool architecture has been reengineered in order to apply semantic mutation operators into the model-driven development process and realize testing on a set of semantic mutants. The tool and the implemented mutation operators have been verified in a case study on a status service for a social network.