To determine whether the potential Tcf/Lef binding site on the endogenous OPN promoter is occupied by endogenous Tcf-4 and β-catenin proteins, a ChIP assay was performed using human breast cancer cell line MDA MB 231, which expresses high levels of Tcf-4, β-catenin and OPN. The gene discussed is SPP1; the disease is breast carcinoma.