Epistasis among HLA-DRB1 alleles in a Spanish cohort with multiple sclerosis - Romero-Pinel, L; Pujal, J; Martinez-Yelamos, S; Gubieras, L; Matas, E; Bau, L; Torrabadella, M; Azqueta, C; Arbizu, T (JOURNAL OF NEUROLOGY, 2010)