Taking into account that FOXJ1 is highly specific of MCCs, and that we observed a good concordance between the number of MCCs detected by scRNAseq and the number of cells expressing FOXJ1 by IHC, we performed a preliminary study of FOXJ1 expression by IHC in order to evaluate the frequency and possible significance of MCC differentiation in BC. The gene discussed is FOXJ1; the disease is breast cancer.