Human rights in the EU, as enforced by the ECJ, are based on the "general principles of EU law" – ascertained by the ECJ on the basis of various sources including national constitutions and traditions – as well as the European Convention on Human Rights (ECHR), and the Charter of Fundamental Rights of the European Union. The status of the Charter was reinforced by the Treaty of Lisbon, which entered into force on 1 December 2009, though the United Kingdom, Poland, and the Czech Republic negotiated a protocol to the treaty which "purports to limit the impact of the Charter in those states".