ERBB2 and breast cancer: Furthermore, they also observed that the miRNA cluster miR-99a/let-7c/miR-125b was predominantly expressed in Luminal A breast cancer (hormone receptor-positive/HER2-negative/levels of the protein Ki-67), its low expression predicting poorer outcomes [193], thus ensuring the distinction between it and Luminal B type tumors (hormone receptor-positive/HER2-positive/HER2-negative with high levels of Ki-67).