The luminal B breast cancers are characterized by higher Ki-67 and lower expression of several luminal-related genes (like ESR1 or FOXA1), genomic instability and a higher frequency of TP53 gene mutations; thus, they are associated with a worse prognosis and a higher risk of relapse than luminal A breast cancers [5, 12]. The gene discussed is MKI67; the disease is breast carcinoma.