This observation is interesting because OPN mRNA expression levels can differentiate aggressive or indolent breast cancers only in patients expressing high levels of Tcf-4 mRNA expression, suggesting that the tumour suppressor function of Tcf-4 may only be expressed in OPN-low background (in the case of Rama37 cells where Tcf-4 suppresses OPN) but not in OPN-high background (in the case of MCF10AT cells where Tcf-4 activates OPN). Here, SPP1 is linked to breast cancer.