P sued D seeking a tax refund because DOMA singled out legally married same-sex couples for 'differential treatment compared to other similarly situated couples without justification.' P was the executor of the estate of her deceased same sex spouse. The couple was married in Canada and resided in New York. P was denied the tax benefit of spousal deduction for federal estate taxes. The court held that DOMA was a violation of due process guarantees of the Fifth Amendment. The United States (D) appealed.