Hence, we investigated the phenotypical and functional characteristics (e.g., tumor grade, mitosis rate, tumor budding, HR and HER2 status, and Ki-67 proliferation index) of two breast cancer cell lines with a) different histological features and b) different biological aggressiveness, namely, the HR+ (luminal) cell line MCF-7 and the more aggressive TNBC-like cell line MDA-MB-231, in both the CAM and mouse xenograft models. The gene discussed is MKI67; the disease is breast carcinoma.