The impact of IHC-defined BC subtypes on TTBM was examined via multivariable analysis in only five studies [58, 70, 84, 90, 94], of which three [70, 84, 90] reported significant multivariable associations (HR:2.0, p = 0.004 for HR-/HER2+, HR:1.51, p = 0.0002 for non-luminal BC subtypes, and sHR: 2.08, p = 0.03 for HR-/HER2-, respectively). Here, ERBB2 is linked to breast cancer.