Uskowi on Iran has a helpful primer:
The only legal course available to Mousavi and his supporters, if and when the Guardian Council rejects his appeal as expected, is the intervention of the Assembly of Experts. The Assembly, comprised of 86 leading Shia jurisprudence experts, has the constitutional power to select, supervise and dismiss the supreme leader.
The supervision and dismissal clauses have never been used by the Assembly, and it is considered very unlikely that the Assembly would now use its constitutional power to force the supreme leader to nullify the election results. What makes it intriguing, however, is the fact that Ayatollah Hashemi Rafsanjani was elected last year as the Assembly’s chairman and has strong support within the 86-person assembly, all clerics like himself. Rafsanjani has emerged as the most powerful opponent of Ahmadinejad and a strong supporter of Mousavi.