To asses in an in vitro model of breast cancer the miRNA-target interaction and its effect on BRCA1, expression levels of ID4 and miR-342 were measured in five human breast cancer cell lines including MDA-MB-231 and BT20 (ER-negative), HCC1937 (BRCA1-mutant, ER-negative) [26], MCF7 and T47D (ER-positive). The gene discussed is ESR1; the disease is breast cancer.