Human breast cancer tissue samples from 18 patients with high-risk or low-risk MammaPrint breast cancer were analyzed. The membrane fractions were enriched, and the extracted proteins were reduced, alkylated, digested with Lys-C and Trypsin, and labelled with iTRAQ4plex: 114; high risk; 115; low risk. Peptide and protein assignments were filtered to a false discovery rates (FDR) of < 1% by searching against a randomized decoy database using Mascot.