Since CTGF is expressed at relatively low level in ERα-positive cell lines, such as MCF7 (approximately 15 ng/107 cells/24 h determined by ELISA) and ZR75-1 (approximately 12 ng/107 cells/24 h) breast cancer cell lines (data not shown), a sensitive ELISA-based protein-protein binding detection method was employed to determine interaction of endogenous CTGF with endogenous ERα. This evidence concerns the gene ESR1 and breast cancer.