A STEER module provides steering, run management, interface classes, and base classes to the entire framework. The detectors are independent modules that contain the code for simulation and reconstruction while the analysis code is progressively added. Detector response simulation can be performed via different transport codes like GEANT3, GEANT4, and FLUKA. The user can decide which one to load at runtime and, thanks to the Virtual Monte Carlo abstract interface, the transition from GEANT3 to GEANT4 and to FLUKA is seamless, since no user code has to be changed; only a different shared library has to be loaded.