ERBB2 and breast cancer: Recently cDNA microarray studies have classified breast cancers broadly into five subtypes, based on their different expression profiles: normal breast-like, luminal A (oestrogen receptor (ER) positive), luminal B (ER positive and proliferative), HER2 overexpressing and basal (Sorlie et al, 2001, 2003), the latter having a particularly poor clinical outcome (Sorlie et al, 2003; Abd El-Rehim et al, 2004; Nielsen et al, 2004).