The patient was characterised with selected clinical and anthropometric features: BMI = 23.98 kg/m2, age = 41 years, she gave birth to one child, without co-existing diseases, T1c, N0, M0, stage IA, invasive ductal carcinoma, histological grade (G2) according to the Elston–Ellis classification, tumour diameter = 1.6 cm, ER/PR+HER2− and low proliferative index; Ki-67 = 5%, molecular subtype: luminal A. The subject was qualified for breast-conserving surgery (BCS). The gene discussed is MKI67; the disease is neoplasm.