In what context are you answering this question? There are two potential approaches I'd consider - or perhaps you might want to use both:
1) Look at EU-specific fundamental rights law in the EU, from the early cases like Internationale Handelsgesellschaft and SPUC v Grogan to the Charter of Fundamental Rights and surrounding cases (note first Fransson, then look at cases like Test-Achats, Digital Rights Ireland and NS v Secretary of State).
2) Discuss Opinion 2/94 and Matthews v UK, and the response to this through Art 6(2) TEU, and thus Opinion 2/13 (on which Sionaidh Douglas-Scott has written an excellent article).
Hope this helps.