ERBB2 and breast cancer: First, we determined MDM2 levels in breast cancers using GTEx normal breast tissue (n = 179), TCGA normal breast tissue (n = 113), and TCGA breast cancer tumor subtypes (luminal A (n = 562), luminal B (n = 214), basal (n = 190), HER2-enriched (n = 81), and normal-like (n = 39).