In the second stage, the selected potential aging miRNAs and candidate reference miRNAs were re-evaluated by individual RT-qPCR in plasma samples from a new, independent study cohort consisting of 10 young (mean age 41 years) and 20 older (mean age 78 years) breast cancer patients, all diagnosed with lymph node negative, luminal A (grade 1–2, ER-positive, PR-positive, HER2-negative) tumors (see Table 2). Here, ESR1 is linked to breast cancer.