GPC1 and neoplasm: Consistent with the strong association of high α3(V) and GPC1 expression with luminal tumours, high expression of the two proteins also associates strongly with oestrogen receptor-positive (ER+) and progesterone receptor-positive (PR+) tumours, but not with human epidermal growth receptor 2-positive (HER2+) tumours (Fig. 10d,e).