PREX1 and breast carcinoma: However, we found that in breast cancer cells the luciferase activity of a PREX1 promoter reporter that includes the Sp1 sites (located in the proximal CpG island, positions -201/-192 and -170/-161) does not change after methylation (data not shown), suggesting that methylation of the Sp1 sites is not relevant for controlling the expression of the gene.