diff --git a/CMake/mitkFunctionAddExternalProject.cmake b/CMake/mitkFunctionAddExternalProject.cmake
index 6325b72bff..d0a3a5c264 100644
--- a/CMake/mitkFunctionAddExternalProject.cmake
+++ b/CMake/mitkFunctionAddExternalProject.cmake
@@ -1,105 +1,107 @@
 #! Register an external project.
 #!
 #! Calling this function registers an external project with the MITK
 #! build system. Usage:
 #!
 #!   mitkFunctionAddExternalProject(NAME name [ON|OFF]
 #!     [PACKAGE package] [COMPONENTS comp1...]
 #!     [DEPENDS dep1...]
 #!     [DOC docstring]
 #!     [NO_CACHE] [ADVANCED] [NO_PACKAGE]
 #!   )
 #!
 #! The function creates a MITK_USE_<name> internal cache variable if the
 #! NO_CACHE option is given. If the NO_CACHE option is not set, a normal
 #! boolean cache option is created with the given DOC argument as help string.
 #! The option is marked as advanced if the ADVANCED option is set. The
 #! option's initial value matches the ON or OFF option and defaults to OFF
 #! if none is given. The DEPENDS arguments are used to force the
 #! corresponding MITK_USE_<depN> cache variables to ON if MITK_USE_<name>
 #! is ON.
 #!
 #! The PACKAGE argument names the argument for the
 #! corresponding find_package() call in MITKs top-level CMakeLists.txt
 #! and MITKConfig.cmake file. It defaults to <name>. If COMPONENTS are
 #! specified, these are passed to the find_package() call. If the
 #! NO_PACKAGE option is given, the find_package() call is suppressed.
 #!
 #! For each registered external project there must exist a file called
 #! CMakeExternals/<name>.cmake which defines the build process for
 #! the external project.
 #!
 #! \note Note that multiple calls of this function must be ordered
 #! relative to their declared dependencies. This applies to the DEPENDS
 #! arguments of this function as well as the actual target dependencies
 #! declared in the CMakeExternals/<name>.cmake file.
 #!
 function(mitkFunctionAddExternalProject)
   cmake_parse_arguments(EP "ON;OFF;NO_CACHE;ADVANCED;NO_PACKAGE" "NAME;DOC;PACKAGE" "DEPENDS;COMPONENTS" ${ARGN})
   if(NOT EP_NAME)
     message(SEND_ERROR "The NAME argument is missing.")
   endif()
   set_property(GLOBAL APPEND PROPERTY MITK_EXTERNAL_PROJECTS ${EP_NAME})
   if(NOT EP_DOC)
     set(EP_DOC "Use ${EP_NAME}")
   endif()
   if(NOT EP_PACKAGE AND NOT EP_NO_PACKAGE)
     set(EP_PACKAGE ${EP_NAME})
   endif()
 
   set(_use_var "MITK_USE_${EP_NAME}")
   set(_on 0)
-  if(EP_ON OR ${_use_var})
-    set(_on 1)
+  if(DEFINED ${_use_var})
+    set(_on ${${_use_var}})
+  else()
+    set(_on ${EP_ON})
   endif()
 
   if(_on AND EP_DEPENDS)
     # Get all transitive dependencies
     set(_depends_all)
     set(_depends_cur ${EP_DEPENDS})
     list(REMOVE_DUPLICATES _depends_cur)
     set(_depends_new ${_depends_cur})
     while(NOT "${_depends_all}" STREQUAL "${_depends_cur}")
       list(APPEND _depends_all ${_depends_new})
       set(_depends_new_tmp )
       foreach(dep ${_depends_new})
         get_property(_dep_dep GLOBAL PROPERTY MITK_${dep}_DEPENDS)
         list(APPEND _depends_new_tmp ${_dep_dep})
       endforeach()
       set(_depends_new ${_depends_new_tmp})
       if(_depends_new)
         list(REMOVE_DUPLICATES _depends_new)
         list(APPEND _depends_cur ${_depends_new})
       endif()
     endwhile()
     # Force dependencies to ON
     foreach(dep ${_depends_all})
       if(NOT MITK_USE_${dep})
         get_property(_depends_doc CACHE MITK_USE_${dep} PROPERTY HELPSTRING)
         get_property(_depends_type CACHE MITK_USE_${dep} PROPERTY TYPE)
         if(NOT TYPE STREQUAL "INTERNAL")
           # This is a cache UI variable
           message("> Forcing MITK_USE_${dep} to ON because of ${_use_var}")
           set(MITK_USE_${dep} ON CACHE BOOL "${_depends_doc}" FORCE)
         else()
           # This is an internal cache variable
           set(MITK_USE_${dep} ON CACHE INTERNAL "${_depends_doc}" FORCE)
         endif()
       endif()
     endforeach()
   endif()
 
   # Set the actual MITK_USE_<name> cache variable
   if(EP_NO_CACHE)
     set(${_use_var} ${_on} CACHE INTERNAL "${EP_DOC}" FORCE)
   else()
     env_option(${_use_var} "${EP_DOC}" ${_on})
     #set(${_use_var} ${_on} CACHE BOOL "${EP_DOC}" FORCE)
     if(EP_ADVANCED)
       mark_as_advanced(${_use_var})
     endif()
   endif()
   set_property(GLOBAL PROPERTY MITK_${EP_NAME}_PACKAGE ${EP_PACKAGE})
   set_property(GLOBAL PROPERTY MITK_${EP_NAME}_COMPONENTS "${EP_COMPONENTS}")
   set_property(GLOBAL PROPERTY MITK_${EP_NAME}_DEPENDS "${EP_DEPENDS}")
 endfunction()