Breast cancer (BC) is the most common tumour in women and the fifth cause of cancer death worldwide.1 BC is a heterogeneous disease at the inter‐ and intra‐tumour level, which is relevant to the prognosis and therapy of the disease.2 The type of BC, its location and other factors (differentiation grade, size, presence of different proteins—Ki67, p53, E‐cadherin—sentinel lymph node, patient age and response to treatment) are also relevant to the prognosis.3 The gene discussed is MKI67; the disease is cancer.