ESR1 and breast cancer: Chlorpyrifos (CPF), an extensivelyemployed biocide, was reported to generate breast cancer after repeatedexposure at low doses in rats1 and to increasethe risk of breast cancer development in women.2 In addition, it was reported to produce cell proliferationin human breast cancer cell lines expressing (MCF-7) or not (MDA-MB-231)estrogen receptor after unique and long-term treatment.3,4 However, to date the complete mechanisms through which CPF couldinduce this effect remain to be discovered.