Although international treaties prohibit particular crimes, it is through the jurisprudence of the ad hoc Tribunals that crimes and various forms of participation as well as defences have been much further defined. Domestic prosecutorial initiatives should pay attention to the significant developments in the jurisprudence dealing with genocide, crimes against humanity and war crimes from ICTY and ICTR. These include significant judgements on the definition of genocide;45 the scope of crimes against humanity and its various underlying crimes,46 including extermination47 and persecution;48 the definition of various forms of sexual crimes, including rape as genocide,49 rape as torture, or enslavement;50 conditions of application of grave breaches of the Geneva Conventions 51 and violations in the context of internal armed conflict;52 forms of participation including superior responsibility,53 joint criminal enterprise,54 and aiding and abetting;55 and the availability of defences such as duress,56 reprisals57 and diminished responsibility.58 The Rome Statute and its Elements of Crimes may provide further guidance. If a crime is not clearly defined in domestic law at the time it was committed, a prosecution may contravene the principle of legality. Other legal obstacles to prosecutions may include statutes of limitations, immunities, double jeopardy or ne bis in idem. These obstacles should be challenged creatively, as has been the case in a number of recent examples.59 45 J elisic, ICTY Appeals Chamber, 5 July 2001; Krstic, ICTY Appeals Chamber, 19 April 2004; Akayesu, ICTR Trial Chamber, 2 September 1998. 46 Tadic, ICTY Appeals Chamber, 15 July 1999. 47 Krstic, ICTY Trial Chamber, 2 August 2001. 48 Kupreskic, ICTY Trial Chamber, 14 January 2000. 49 Akayesu, ICTR Trial Chamber, 2 September 1998. 50 Kunarac, Kovac and Vukovic, ICTY Appeals Chamber, 12 June 2002. 51 Tadic, ICTY Appeals Chamber, 15 July 1999. 52 Tadic, ICTY Appeals Chamber, 2 October 1995. 53 Blaskic, ICTY Appeals Chamber, 29 July 2004. 54 Tadic, ICTY Appeals Chamber, 15 July 1999. 55 F urundzija, ICTY Trial Chamber, 10 December 1998. 56 Erdemovic, ICTY Appeals Chamber, 7 October 1997. 57 Kupreskic, ICTY Trial Chamber, 14 January 2000. 58 Vasilijevic, ICTY Trial Chamber, 29 November 2002. 59 International developments over the past few decades may be helpful. For instance, on statutes of limitation, see the Convention on the Non-Applicability of Statutory Limitations to War Crimes and Crimes against Humanity. On immunities of former and current Heads of State, guidance may be found in the statutes of international criminal courts and in the decision on the lack of immunity of Charles Taylor (decision by the Special Court for Sierra Leone, 31 May 2004), Jean Kambanda before ICTR and Slobodan Milosevic before ICTY. For guidance on ne bis in idem, see article 20 of the Rome Statute. For a very useful discussion on these issues as well as other matters of State practice, see the “Independent study on best practices, including recommendations, to assist States in strengthening their domestic capacity to combat all aspects of impunity” by Professor Diane Orentlicher (E/CN.4/2004/88). This study was aimed at commenting on the Set of Principles for the protection and promotion of human rights through action to combat impunity (E/CN.4/Sub.2/1997/20/Rev.1, annex II), which have since been updated (see E/CN.4/2005/102/Add.1). 22