Prognostic significance of CD9 expression differs between tumour cells and stromal immune cells, and depends on the molecular subtype of the invasive breast carcinoma.

T-CD9 expression could be a biomarker for poor prognosis in luminal A IBC, whereas S-CD9 expression could be a marker of good prognosis in luminal B (HER2-negative) IBC. Therefore, tumour compartment-specific analyses considering molecular subtypes are necessary to study the prognostic significance of CD9 expression in IBC.