The IRS has issued Notice 2017-15 which provides relief for estate or gift tax transfers to same-sex spouses.
The Supreme Court had declared DOMA (Defense of Marriage Act) unconstitutional on June 26, 2013 (U.S. vs Windsor).
Revenue Ruling 2013-17 allowed taxpayers in same sex marriages, recognized under state law, to file original or amended returns to claim credits or refunds for any estate or gift tax returns filed that did not utilize the unlimited marital deductions on transfers to same sex spouses. This was only allowed prospectively and only where the statute of limitation had not expired.
Under the recently issued Notice 2017-15, taxpayers are not barred from amending prior estate or gift tax returns in order to apply the unlimited marital deduction, thereby freeing up additional exemption for future transfers. The statute of limitations for amending does not apply for this purpose. However, refunds or credits can not be issued if the statute of limitation expired.