In 1998, Devon A.Thomson and Ronald J.Weigel of Stanford University in the United States used suppression subtractive hybridization (SSH) to screen the homologous gene with Xenopus anterior gradient-2 (XAG-2) from the cDNA library of estrogen receptor positive breast cancer cell line MCF7, named hAG-2, or anterior gradient-2 (AGR2) (1). Here, ESR1 is linked to breast cancer.