code: @Override
  	public void setEntityReference(EntityReference entityReference) {
  		if(entityReference instanceof ProteinReference || entityReference == null)
  			super.setEntityReference(entityReference);
  		else
  			throw new IllegalBioPAXArgumentException("setEntityReference failed: "
  					+ entityReference.getUri() + " is not a ProteinReference.");
  	}
docs: ------------------------ INTERFACE METHODS ------------------------