In order to examine the effects of phenol red on the stemness of ER-positive human mammospheres (MCF-7, M13SV1, M13SV1 R2, M13SV1 R2N1), we measured the cancer stem cell marker, OCT4 gene expression, in mammospheres cultured in phenol red-free or phenol red-containing MEBM. This evidence concerns the gene ESR1 and cancer.