ESR1 and breast cancer: It is noteworthy that both miRNA expression levels showed a significant negative correlation with ERα immunoexpression, with the highest miRNA expression level in CMTs having the lowest ERα immunoexpression, and although it has long been known that human and canine mammary neoplasms lose ERα expression along with increased stage and grade, our findings may indicate that miR-18a and miR-18b contribute to a loss in hormone receptor activity [24,25].