In this study, we found the expression of the microRNA miR-550a-3p was negatively correlated with protein levels of ERK1 and ERK2, two pivotal effectors in the oncogenic Ras/ERK pathway, and ascribed significant diagnostic and prognostic values to the downregulation of miR-550a-3p during breast cancer initiation and progression. The gene discussed is MAPK3; the disease is breast cancer.