We investigated inter- and intra-laboratory variability in the assessment of ER, PR, HER2, Ki67, and histological grade in a non-selective population-based nationwide cohort of 43,261 invasive breast cancer patients diagnosed between 2013 and 2018, using real-world data from the Swedish National Quality Register for Breast Cancer (NKBC). The gene discussed is MKI67; the disease is breast carcinoma.