High EGFR expression in the HER2+ breast cancers was significantly associated with high MPAS (ρ = 0.42, P = 7 × 10−14), while HER3 and p27 expression were negatively associated with MPAS (ρ = −0.21, P = 6 × 10−4 and ρ = −0.11, P = 0.076, respectively) in this subgroup, implying that higher relative expression of EGFR to ERBB3 may be responsible for the higher MAPK signaling and poorer survival outcomes. Here, ERBB3 is linked to breast cancer.