NUMB and breast cancer: We utilized a BC case cohort (N = 890), previously employed to establish the role of exon 3 loss and NUMB hyper‐phosphorylation as causal events in BC (henceforth, the European Institute of Oncology (IEO) cohort).[1b,e] Within this cohort, we conducted a comprehensive analysis of NUMB expression on whole formalin‐fixed paraffin‐embedded (FFPE) slices using immunohistochemistry (IHC) and RNAseq.